Crazy combinaiton idea coming (which of course will probably be less crazy than Espithel's idea...) Maybe a Fuse-card (my previous idea for mechanic), with a create-creature ability which in turn has a Counter Tacticswhich turns into something else.
Like, (bad example) Knight, target a Pegasus to turn into -> Pegasus Rider, has ability "Create 'Young Pegasus' -> Yong Pegasus, ability Learn: when opponent plays a creature with an ability, lose this ability to learn that ability instead"
Yeah, flavour-wise it doiesn't really make sense, but you get my point.
Maybe this is a bit too much, (but I am not sure if you can do this challenge without making it "too much")
Another idea: Creature generator which creates creature A that has an ability to turn it into creature B, and also turns into creature C on death.
or: Card with an ability that can turn into 3 different creatures depending on certain conditions, like turn into A if your HP is more than opponent's, int B if it's less and into C if equal