Definately T3 and Hk
I really think that Canderous should return for a short portion of the game, but not join your party. Maybe lead an army of Mandalorians into battle at some point in time.
I also would like to see Jolee and Kreia in some form or another at least for one scene. (preferably the same scene where the two of them start arguing over ideas and such... I at least think it'd be funny
Also a Carth and Atton dialogue would be nice too
Bastila so long as you determine somewhere in the game whether she died in the first game or not.
And of course Revan and the exile for the ending of the game.
Oh, and even though he wasn't actually a main character at all, I'd like to see that Iridorian guy that you meet in the first game on Manaan, the one in the body armor. It'd be fun if he could join your party in the third game, in my opinion