Well HK and T3, obviously! :)

Other than those two, I think only Visas and the Handmaiden should make a re-appearance, although I don't know about Visas after the Ravager, though. As like, she's dead.
That is a choice though,you could just say that it didn't happen.

Revan would be the main PC, Exile, Mandalore, HK-47, T3-M4, Bastila, Jolee, Atton should all be party members Yes I know the issues with some of that and there would be new party members ofcourse. All characters that didnt die would play a role in the game especially Carth and Bao Durr working to help the Republic.

I voted Bao-Dur and Visas. Bao because I can only begin to imagine the shenanagins he and a new remote might get into...

Remote 2.0-Beeep. Boop-dee. BEEET! (What's up Bao, you look sad)
Bao- I kinda miss my old remote. We went through everything together, and you kinda suck.
Remote 2.0- BEEEP. Boo-deet. Deeeeeet dee-beep. BEEP! (So you can replace your freggin arm and still be content, but your old remote gets blown up with a wasteland planet and you go emo? You suck!)
Bao- JUST. SHUT. IT. ~cries~
(That was mean of me. SORRY BAO! ~pets Bao-Dur avatar~)

Visas because I would like to see how she goes on without a master and Post-Vacation-to-Kataar.

I dont really mind to be honest just along as kreia is wiped form star wars history!

I picked Mira because I figured that the Droids (T3 & HK) would already be in part III by default :D And I found Mira to be quite useful, and an interesting character.