Well heres part of the reason I think TSL ending is soo poo, as when Obsidian made TSL there are alot of Charachters you can't bring back due to what could have happened to them in the previous game.
Hk-47 and T3M4 will be in it as they are the C3PO and R2D2 of KotOR
Bastila would be my wish, but her fate is open ended. So don't know if we would see her making anything but another cameo appearance.
Carth Onasi he's a legend! And he has to be alive as he ran off on the Unkown World. Would be good to have him around again, or maybe at some point you could be on his ship.. he is an admiral now you know
Bao Durr, I know some don't like him, but I think the guy is a legend! Quiet, understated but gets the job done.
Atton is another cool one. For me he is the Han Solo of KotOR.
Revan, well Revan is Revan so enough said there
Mira, the only person in the whole 2 games I would wanted romantic options with and they weren't there... ah well!