cool, i was thinking a pure spellcaster, maybe an ur-priest of some sort. although i could play something else, another thought i had was a dark stalker from Fiend Folio. they're humanoid appearing, so they could blend in. although a doppleganger would be interesting if you'd allow it, a doppleganger psion (hehehe) or, what's that shape changing PrC from CW at the very end of the prc section. hehe... if i went with the doppleganger psion i'd focus on domination and things like that, a manipulator. as a dark stalker probably end up as a rogue-ish type character, although you seem to have plenty of those already. as an ur-priest, well that'd need some mental fleshing out in my head, a wizard/cleric/ur-priest/MT could have a great deal of diversity to wreak some havoc.
for personality, i've been wanting to play a character based off of the darkest readings of Nietzsche... an uberman, forges his own destiny without pity or regret, considers mankind a flock, a sharp mind and body, treats those who can't forge their own destiny with vile disdain
that could work with a psionic-dominator type character (the sheep are my toys, he'd end up a bit of a hypocrit), or an ur-priest (only sheep follow gods, literally the anti-christ

), or a monk (i need no sheep forged weapon).