TSL had 3 returning characters and I think its safe to say that KOTOR 3 will probably include some as well so I wanted to discuss who I thought should make it in.
I think T3-M4 and HK-47 are shoe-ins. Their obviously the new R2 and C3 and are going to find a way to weave themselves into the story line no matter what.
Next I want to consider the characters from KOTOR.
Bastila - I think Bastila would make a great returning character. She wants to find Revan and so she has motivation to go with whoever our character ends up being. Her issues aren't all resolved; for instance she'll be dealing with self-doubt about giving in to Malak and will be worried she can fall to the dark side again. With the advent of the influence system of TSL all our characters basically have to be morally ambigous and Bastila fits the bill. Only catch is you could have killed her in KOTOR but it doesn't appear to be cannon based on her cameos in TSL.
I don't think Carth would be so good. His problems are all resolved and he has responsibilities with the Republic fleet. I always like a cameo though. Mission's out because she can be killed. Juahni is probably dead on Katar. While Jolee Bindo is one of my favorite characters I doubt he'll make a comeback. Lastly of the KOTOR characters despite Canderous/Mandalore's return in TSL, I didn't find him to be a strong character and so I'd be surprised if we see him a third time.
Of the TSL characters Kreia and G0-T0, either Hanharr or Mira is dead, Bao-Dur's death is implied but uncertain . I think several others were supposed to die as well before the cuts. I doubt any of the jedi trainable characters will be back because to make them jedi you have to reveal their entire storyline and resolve their issues. Visas' storyline seemingly ends with the death of Nihilus. I think the best chance for a return is Disciple/Handmaiden based on whether you indicate that the exile is male or female.
Sorry that was long. I'd like to see who everyone else would like to see back in KOTOR 3.
edit: oh yeah and I forgot the wookies. I wouldn't be surprised to see either Zaalbar or Hanharr make a comeback depending on whether or not you killed Hanharr in TSL. You have to have a wookie but I don't think were going to see a third new one.
Mod Edit: Merged repeat thread with pre-existing like thread. For tidyness sake. -RH