Something quick.

Knuckles as Ansem. His garb is a mixture of Ansem and Enerjak's.

Those are Cores/Hearts floating behind him as he wanders the realm of darkness.

Yeah his eyes are slitted.

Long story short:

Ventus - Sonic
Terra - Knuckles
Aqua - Tails
Eraqus - Edmund
Xehanort - Dimitri

Sora - Silver
Riku - Shadow
Kairi - Blaze

Ansem the Wise - ???