This idea is pure genius.
I love adding extra thought into the game, and this makes sense, and is just plan cool.
However, it could end up as a nerf to reverse time/rewind, unless you use it in combination with other things.
I dislike the idea of shrieker turning back into graboid though, that although logical would remove one of the downfalls of graboid evolution, causing them to play
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rather than 8
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.
Skeleton + Reverse time to something living is great though.
Nymph -> Fate Egg also seems pretty strong, since they can never get
that nymph back.
Ash Eater -> Ash = Win. (maybe it should turn into the upped counterpart, and Brimstone eater turns to the upped)
I love these ideas. A little more thought, just to keep the games interesting, and separate the good players from the grindy ones.