Non-playables:
Revan
Bastila
Playables:
T2-M3 and HK-47 is a given, theyre the c3po and r2d2 of kotor.
...and:
Yuthura Ban! that'd be really cool.
The rest can be new as new characters provide more new stuff to the storyline.
I'd make an exception for Visas, as she and her hot lips could be playable as well.
But Visas only loves the exile, and shes only cute if shes in love with the PC, so then she couldnt be cute anymore unless the exile is the main character of kotor III as well. Which wouldve been wrong! So ive changed my mind!
Visas should be one of the important NPCs. Maybe you could go to Katarr to find her or something.