Hi, due to a horrible lack of free time, I haven't posted anything in a while, but I'll take advantage of some unexpected time to post three cards which I've given some thought to. My english is not good at all (spanish, german and arabic I speak better, so english would be my fourth languaje). Because of that, my card names and flavour text are pretty bad (to the point where I basically give up on trying to come up with any, and just give you a basic idea of what Im looking for, and let you decide). Anyway, heres a card I think would get a lot of play in T1, as well as in other formats.
Mana Recicler (Got a good name?)

Enchant Creature
When enchanted creature is put into a graveyard from play, add mana equal to that creature's mana cost to your mana pool.
"Insert flavour text here.... (Probably something about the essence of living things continuing to exist and blah blah)
The flavour is something along the lines of a creatures soul, or essence continuing to linger after death, or maybe something like regrowth, but definately green. It started out costing 1G, but seeing that it makes card disadvantage, being a creature enchantment, I decided to cut it down, I really can think of it being broken, because if you have an expensive creature out anyway, you could just win with it, and if your opponent has it youd want to destroy at as well, and using two cards to generate mana isnt that big of a deal, especially since the creature being destroyed would have to cost more than the destroy spell and this.... well Ill let you decide about the card.
Remember it can be slapped on an opponents creature as well. Has potential to be abused, by goblin welder for example (although there are so many better ways to abuse welder) or FCG. This produces coloured mana equal to the creatures cost.
Sorry for not putting in any text.
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Current Version
Energy Conservation

Enchant Creature
When enchanted creature is put into a graveyard from play, add mana equal to that creatures mana cost to your mana pool and return Energy Conservation to your hand.
""In death their memories of the land became our memories of the land"