I think the third ability is a very key part of the card actually. Let's say the card was just this, as you suggest (which is how I originally conceived it):

Embers of Remembrance
 
Sorcery
Return target Red card with converted mana cost X from your graveyard to your hand.
Embers of Remembrance deals X damage to target creature or player.

There is an obvious power level problem with this. With one of these in the yard and one in your hand, it effectively becomes a sorcery speed Searing Touch with buyback for  , because this card's converted mana cost is 1. Seems too good, especially with what else the card does for you by providing red some card advantage. So the solution is to have it remove itself. But, that got me to thinking that I didn't want to remove the Searing Touch aspect of it entirely. I did this by adding a pseudo-kicker to the removal clause. Sorcery speed Searing Touch w/buyback for    is much more fair, and still rather good actually. You can cycle between 2 of them for 4 a turn to wear down your opponent, and then break the loop by using it on a larger burn spell to finish them off.

Embers of Remembrance
 
Sorcery
Return target Red card with converted mana cost X from your graveyard to your hand.
Embers of Remembrance deals X damage to target creature or player.
Remove Embers of Remembrance from the game unless you pay  .

A powerful card, but with the limitation that the two effects are tied together very closely.

Loam Transmuter
 
Creature - Elemental
  : Remove up to three cards in target opponent's graveyard from the game.
  , Remove three cards in your graveyard from the game: You gain 3 life, draw a card, and put a 1/1 green Saproling creature token into play.
3/5

I'd like to see more green creatures along these lines -- well costed, but that can actually do interesting things and aren't just large idiots.

I've modified it so that the abilities and life costs line up with how in-color they are. This version makes more sense as a mono-black card; the costs to get off color stuff are steeper and otherwise it's a good Nantuko Husk/Giant Cockroach.

Symbiotic Exoskeleton
 
Enchantment - Aura
Enchant creature
Enchanted creature gets +2/+2.
: Regenerate enchanted creature.
When Symbiotic Exoskeleton is put into a graveyard from play or from the stack, draw a card.

They've been trying to make creature enchantments playable and not inherently card disadvantage with varying success for a while now. This is one execution that when I thought of it I was actually shocked it hasn't been done yet. The fact that it deals with either itself being countered or the creature being killed in response, or the creature eventually leaving play is fitting for green I think. I know I'd want to play with this.


Current wording:
Symbiotic Exoskeleton
 
Enchantment - Aura
Enchant creature
Enchanted creature gets +2/+2.
: Regenerate enchanted creature.
When Symbiotic Exoskeleton is put into a graveyard from play or from the stack, draw a card.

Cyclical Unconscious
 
Sorcery
Remove Cyclical Unconscious from the game and reveal your hand. If there are seven or more creature cards in total among your permanents, cards in your graveyard, and cards in your hand, discard your hand. If you did, then you may shuffle your graveyard into your library. Then you draw seven cards.

Collective Unconscious never really was good was it? Now they've made Harmonize which is obviously miles better, so why not push it even further? Here's a Green personal Timetwister/Wheel of Fortune. The fact that you get either effect feels very green, and so does tying it to creatures and having it affect only you. I really like this one and I hope you all do too.

Saurian Sustainer
[2]GG
Creature - Lizard Shaman
Spells and abilities your opponents control that target Saurian Sustainer cost an additional [3] to play.
[1]G, Sacrifice two lands: Regenerate target creature. If it's regenerated this way, put a +1/+1 counter on it.
4/4

Here's an idea for a green 4-drop that's obviously a twist on Troll Ascetic...I made this because I think Troll Ascetic is too good, but at the same time Troll Ascetic is also very much the kind of card that Green needs. This twist on its untargetable ability is less unfair.


Current Wording:
Saurian Sustainer
[2]GG
Creature - Lizard Shaman
Spells and abilities your opponents control that target Saurian Sustainer cost an additional [3] to play.
G, Sacrifice two lands: Regenerate target creature. If it's regenerated this way, put a +1/+1 counter on it.
4/4

Yes, the snake is supposed to be able to get the Provoke and basilisk ability if you entwine it. How could it be worded to make that possible?

To clarify, the Snake token stays in play, but it only has haste and first strike the first turn it's in play.

Rattle of the Viper
[1]GG
Instant
Choose one -- Put a 2/1 green Snake creature token into play that gains haste and first strike until end of turn; or choose a creature you control, and until end of turn, that creature gains Provoke and has "Whenever this creature deals combat damage to another creature, destroy that creature."
Entwine [1]G
Flashback [1]GG and discard two cards

Planar Chaos just gave us a take on Green creature removal in Utopia Vow, so I thought something that suits green's more aggressive side would be interesting.
Definitely Green should not get unconditional creature removal though. Also, I'm an advocate of Green having first strike when it makes sense, as I think this does.

Archmages' Academy
Land
Tap: Add [1] to your mana pool.
Tap, reveal two enchantment cards in your hand: Add one mana of any color to your mana pool.
Tap: Draw a card. Play this ability only if there are exactly seven cards in your hand and the total converted mana cost of enchantments you control is at least 3.

I think that the flavor of knowledge, wisdom, and power in Library of Alexandria combines well with having to be used in a deck built around enchantments. Would a remake like this be printable?

Entropic Fiend
1B
Creture - Horror
Fear
Whenever Entropic Fiend deals damage, you lose that much life unless you sacrifice a creature or a land.
Pay 2 life, sacrifice a creature, sacrifice a land: Target player loses 2 life.
2/1

Here's a guy that that's a throwback to Black's suicide creature flavor, which we have not seen as much of lately. He also gives Black aggro some reach that's in flavor for it.

I think now, with the double green requirement and the lands requirement bumped up to six, it is good, but still fair.

Hey, I realize that I missed the deadline for this challenge due to not having seen it in time, but I wanted to show what I came up with, since I got a real kick out of this. It's true, restrictions do breed creativity! I'll definitely get in on the next one.

Card 1 (White common): "Creature removal"-style creature aura.

Luminous Binding
[2]W
Enchantment - Aura
Enchant creature
If enchanted creature would deal damage, you may gain that much life instead.
[1]W: Tap or untap enchanted creature.
[2]W: Return Luminous Binding to its owner's hand.

Card 2 (Black common): Instant or Sorcery. No discard or creature removal.

Putrid Seism
[1]BB
Sorcery
Destroy target land. You may pay 3 life. If you don't, that land's controller loses 3 life and may search his or her library for a land card, reveal it, and put it into their hand. If that player does, he or she shuffles their library.

Card 3 (Blue uncommon): A "quirky" counterspell.

Spell Schism
[1]UU
Instant
Kicker [2]U
Counter target spell unless its controller pays [3]. If it's countered this way and you paid the kicker cost, choose one -- search its controller's library for up to three cards with the same converted mana cost as that spell and remove them from the game; or search your library for a card with the same converted mana cost as that spell, reveal it, and put it into your hand. If you search a library this way, its owner shuffles it.

Card 4 (Artifact uncommon): Artifact Creature with an interesting form of evasion.

Tsabo's Automaton
[3]
Artifact Creature - Construct
Tsabo's Automaton gets +1/+1, can block two additional creatures each combat, and can't be blocked except by three or more creatures as long as an opponent controls an artifact or land with an activated ability that isn't a mana ability.
Sacrifice three lands: Regenerate Tsabo's Automaton.
2/2

Card 5 (Red rare): Legendary Creature -- Dragon.

Ugunnaz, Vainglorious Tyrant
[4]RRR
Legendary Creature - Dragon Lord
Flying
At the beginning of your upkeep, if you control no lands and no other creatures, you may have Ugunnaz, Vainglorious Tyrant deal 5 damage to target creature.
Sacrifice a mountain or a red creature: Ugunnaz deals 1 damage to target creature and 1 damage to you.
Sacrifice a mountain or a red creature: Ugunnaz gains Haste or First Strike until end of turn and deals 5 damage to you.
Sacrifice seven mountains and/or red creatures: Return Ugunnaz from your graveyard to play.
5/5

Card 6 (Green rare): Splashy instant for Timmy.

Primeval Awakening
[4]GGG
Instant
Primeval Awakening can't be countered.
If it's not your turn, you may sacrifice three forests rather than pay Primeval Awakening's mana cost.
Untap up to three creatures you control. Until end of turn, those creatures get +3/+3, have trample, and can't be the target of spells or abilities your opponents control.

Based on responses, I've upped the power level of the card. What does everyone think now?

Multani's Hymn
1G
Sorcery
You can't play Multani's Hymn unless you control a creature and/or you control five or more lands.
Draw two cards.

I thought it was interesting that although Green is supposed to be the #2 card drawing color, there aren't really any Green draw spells of any worth. (Life from the Loam excepted). So I wondered what an actual basic Green draw spell would look like, and this is the first thing that came to mind. Tying Green's card drawing into the things it naturally cares about seems to make sense.

CURRENT WORDING:
Multani's Hymn
GG
Instant
You can't play Multani's Hymn unless you control a creature and/or you control six or more lands.
Target player draws two cards.

Hurkyl's Rebuke
1U
Instant
You may remove a blue card or an artifact card in your hand from the game rather than pay Hurkyl's Rebuke's mana cost. If you do, Hurkyl's Rebuke can't target nonartifact spells.
Counter target spell unless its controller pays [2].

I wanted to explore the design space of alternate costs that lead to alternate effects, whether restricted, enhanced, or just different.
Here's another example of this, on a permanent rather than a spell, and with alternate MANA cost instead:

Yavimaya Baloth
4G
Creature - Beast
You may pay 1GGG rather than pay Yavimaya Baloth's mana cost. If you do, Yavimaya Baloth comes into play with a +1/+1 counter on it.
4/4

I think this concept has tons of room to explore. Thoughts?

I wanted to make my initial version more powerful, but erred on the side of caution.

Mirrorstone Valley
Land
When Mirrorstone Valley comes into play, target opponent may search his or her library for land card, remove it from the game, then shuffle his or her library.
As long as that card remains removed from the game, its owner may play it. (Even if Mirrorstone Valley isn't in play.)
Tap: Add [1] to your mana pool.
Tap: Add one mana of any color to your mana pool. Mirrorstone Valley deals 1 damage to you.

Here's my take on a rainbow painland. Is the drawback enough to justify this? It doesn't give your opponent a land directly, but it decreases the chances that they will miss land drops, and may actually be enough to lose you the game sometimes.



Current Wording:
Mirrorstone Valley
Land
When Mirrorstone Valley comes into play, target opponent may search his or her library for a land card with a basic land type, remove it from the game, then shuffle his or her library.
As long as that card remains removed from the game, its owner may play it. (Even if Mirrorstone Valley isn't in play.)
Tap: Add one mana of any color to your mana pool.

Yeah I realized this was too powerful with the cantrip. Initially, I figured that just makes it a 1-for-1 counter, since you trade this for your opponent's spell, then you break even on cards since they draw on their extra turn. But it still seemed too good. Cycling this away when you don't need it is an acceptable substitute.

Time Hiccup
U
Instant
Counter target spell.
Draw a card.
You skip your next turn.

Here's my attempt at making a balanced one-mana hard counter. Personally I can't tell if it's really good or really terrible, so hopefully it's neither and is fine. This also definitely needs a better name, but Time Hiccup's OK for a working title.

Current Wording:
Time Hiccup
U
Instant
Counter target spell.
You skip your next turn.
Cycling U

Wow, this is cool. I hit the nail on the head.

Centaur Shieldmage 1GWU
Creature - Centaur Wizard
When Centaur Shieldmage comes into play, you may put a Shield counter on it.
As long as Centaur Shieldmage has a Shield counter on it, creatures you control with converted mana cost 2 or less are indestructible and can't attack or block.
3/4

This is for decks that might want to protect their small utility and mana creatures. It also combos well with mass destruction. The 3-colored casting cost keeps it in check enough I think.


Current Wording:
Centaur Shieldmage 1GW
Creature - Centaur Shaman
Creatures you control with converted mana cost 2 or less are indestructible and can't attack or block.
3/4

As far as it being out of color, I don't think it is...Haste is Red, and Vigilance is White. The double blocking has been historically red or white. Just off the top of my head, there's Two-Headed Giant, Two-Headed Dragon in Red, and Foriyisian Brigade and Entangler in White.

Foriyisian Legionnaire
RRWW
Creature - Giant Soldier
Haste, Vigilance
Foriyisian Legionnaire can block an additional creature.
4/4

As long as we're all in nostalgia mode for Time Spiral... so how about Two-Headed Giant as a Soldier? His soldier training made him much more efficient at attack and defense, obviously (Haste, Vigilance). And this is just one, imagine a whole Legion of them coming at you.
All the abilities are completely centered on creature combat, which is very red-white.

I think it should cost 1RG because the land search effect is not mono-red. If you go with that, I suggest Gaea's Vengeance for the name. Very cool card.

There's so much space to do things with Echo if the costs are changed that it's practically a completely different mechanic.
Just another example:

Prismatic Beast
4G
Creature - Beast
Echo - 3G
When Prismatic Beast comes into play, if you played it from your hand, you may add WUBRG to your mana pool.
4/4

It filters five mana into all 5 colors, so it doesn't add any mana. But, this being a green creature, the cost you actually "pay" for the creature makes it a 4/4 for 4.

Okay, I have quickly realized the power of the current card, and I'm now thinking it should be the simplest spell possible. So discard 2 lose 2 it is.

I definitely think there's a lot of unused space in Echo. I agree that simply having Echo costs be different from the casting costs opens up a LOT of space.

Old echo cards could be reworded like so:
Albino Troll
1G
Creature - Troll
Echo - 1G (rather than simply "Echo")
1G: Regenerate

Here's some very simple examples of new cards created by separating Echo from the mana cost of the card.

Lesser Acridian 
1G
Creature - Insect
Echo - G
2/3

And from there it's a short leap to nonmana Echo costs:

Echoing Fiend
1B
Creature - Horror
Echo - Reveal your hand and discard a card
When Echoing Fiend comes into play, target player reveals his or her hand. Then that player discards a card.
2/1