Risen Martyr, Glib Stratomancer, Crypt Warden...

[Marco]

I am entertaining the idea of changing my rebirth keyword ability into an activated ability. I think this will make it too much like unearth, but am willing to read some other perspectives.

***Keyword Ability: Rebirth***

Rebirth is an ability that enables you to cast certain creature cards from your graveyard.

Undying Phoenix
2RRR
Creature – Phoenix (Rare)
4/4
Flying, haste
RRR: Remove all death counters from Undying Phoenix.
Rebirth 2RRR (2RRR: Return this card from your graveyard to the battlefield with a death counter on it. If this creature would leave the battlefield, if it has a death counter on it, exile it instead. Rebirth only as a sorcery.)

The official rules for rebirth are as follows:

702.XXa Rebirth is an activated ability that functions while the card with rebirth is in a graveyard. "Rebirth [cost]" means "[Cost]: Return this card from your graveyard to the battlefield with a death counter on it. If this creature would leave the battlefield, if it has a death counter on it, exile it instead of putting it anywhere else. Activate this ability only any time you could cast a sorcery."

[Marco]

With rebirth as an activated ability, I could add one or more of the following hosers to my set:

Vigilant Bailiff
1WW
Creature – Human Knight (Rare)
2/2
Activated abilities of cards not in play can't be activated.

Or

Nullstone
4
Artifact (Rare)
Activated abilities of cards not in play can't be activated.

Arcane Initiate
1UU
Creature – Human Wizard (Uncommon)
2/2
T: Counter target spell or activated ability unless its controller pays 1. (Mana abilities can't be targeted.)

Entranced Shaman
RR
Creature – Human Shaman (Rare)
2/1
Sacrifice Entranced Shaman: Until end of turn, other players can't play cards from their graveyards, and you may play cards from other players' graveyards as though they were in your graveyard.
Rebirth 3R (3R: Return this card from your graveyard to the battlefield with a death counter on it. If this creature would leave the battlefield, if it has a death counter on it, exile it instead. Rebirth only as a sorcery.)

[Marco]

Updated rebirth based on feedback from AmbivalentDuck. Rebirth cost is now about 2 mana more than casting cost...

For example:

Arboreal Guardian
G
Creature – Elf Druid (Common)
1/1
Sacrifice Arboreal Guardian: Search your library for a basic land card, reveal it, and put it into your hand. Then shuffle your library.
Rebirth 2G (You may cast this card from your graveyard for its rebirth cost. If you do, it enters the battlefield with a death counter on it. If this creature would leave the battlefield, if it has a death counter on it, exile it instead.)

Vengeful Aven
2W
Creature – Bird Soldier (Common)
2/2
Flying
Vengeful Aven has first strike as long as it has a death counter on it.
Rebirth 4W (You may cast this card from your graveyard for its rebirth cost. If you do, it enters the battlefield with a death counter on it. If this creature would leave the battlefield, if it has a death counter on it, exile it instead.)

Updated first post.