not sure bout sizes as don't play
WHFB but i think it needs to be bigger for a demon prince. have to keep things either avian like the current greater daemon or wierd changy things like horrors/flamers. maybe build a huge flamer-like thing, with more control over its ever changing body (hands morphed into weapons, many faces/heads but can still tell it is a leader, not a giant chaos spawn)
oh wait, tzeentch is supposedly the master of mutation and change, whack a large blob of silly putty or play-doh on a 50mm base and claim it is a daemon prince, can then change its form every turn
lol