After putting some thought into this, I've come up with an idea. Not sure how practical/useful it would actually be in the game, but might be interesting (hope the mechanic is unique enough :s)
Pyramids
permanent with counters/limited uses/chance not to work/other.
ability is to summon a specific/random creature which you'd find there, i.e. either of the following:
skeleton, Mummy, Pharaoh, Anubis, Scarab, Ghost of the Past, Schroedingers Cat(?), Scorpions/only specific ones(?), Immortal(?), perhaps even Iridium Warden(?), others?
As it stands, too many possible creatures I think, just listing them all for options. Another thing is what element the ability cost should be...?
Think it could be quite fun having a permanent which summons creatures thematically