ZombieDemon
A typical Zombieman at first glance, but when it sees you, it morphs into a giant Cyberdemon!
I thought you could use the Zombieman sprites in the 'Spawn' state, then the Cyberdemon sprites in the rest.
Icy Touch
This is a wizard that freezes himself when attacked, to protect from any further damage. Then, he bursts into chunks!
It doesn't make sense, but I thought you could use A_FreezeDeath or something in the 'Pain' state.
Monster Clip
Dropped by a Zombieman, this Monster Clip morphs into a Shotgun Guy upon sight.
Same as the ZombieDemon, but different.
I thought these would be pretty cool. Anybody who wants to make these can.