Love the idea for new class, it sounds really fun and unique.
Some examples:
Lich
Appearance: The player's flesh rots away and is replaced by a dark flowing robe.
Role: Magic damage dealer (player is weak to fire in this form)
Lamia
Appearance: The player's lower body becomes snake-like and thier hair is replaced with slithering snakes.
Role: Crowd Control (sleep/ charm), and Melee Damage over time Dealer (daggers causing bleeding wounds and snake-hair bites/poison)
Demon Wall 
Appearance: The player becomes a large wall with their face sticking out. Movement is restricted to one direction and speed reduced by 95%
Role: Crusher