To clarify exactly what my position is just a few things first...
1. If it is to be KOTOR III, then there must be knights...whether this is kinghts of the old order (represented by KOTOR I and II) or knights of a new order(the lost jedi idea from KOTOR II) that remains to be seen.
2. Since they are Knights, there should be a council - again the compostition (light/dark and who) can be worked out several different ways (some of the other posters here (silent scope for one) have pointed out very easy ways to determine this).
3. As we have seen with the Jedi in all of the movies and games to date, there is the council and then those that have not earned enough favor to sit on the council. And only in times of great need are those on the council willing to get out there and risk their own butts(and sometimes not even then(cough, mandolarian wars, cough))!
That being said, I think that Bastilla will have a role and like others on this post - I do not think that it would be too hard to imagine that the developers of the game make her roles in KOTOR I and II canon - leaving her alive for KOTOR III. That being said, considering the importance of Revan, I would think that Revan's companions would definetly have favor with the new council. I think that Bastilla would have a seat if she choose to accept it (whether she does or not may be up in the air). So I think that would eliminate her from a more interactive role with a new PC.
My idea for a mentor of your new PC, would be to make them an individual that has lost favor with the council...I have always envisioned that at least Mical/Brianna, Atton and Bastilla would sit on the council. So who would be a Jedi that posibly would have lost face with this council? I think that a great story idea (only possible by creating canon from KOTOR II possibilities), would be to bring Atris back as a redeemed Jedi who is determined to be a part of the necessary actions to help the galaxy. She would be a great mentor, that would have a lot of great back story dialogue options (considering she was once the Jedi Historian, she has got to have a few stories floating around in that head of hers) - and I have always pictured her as a weaker fighter (due to the fact that she choose to hide in the past and "learn"), so I don't think that it would be too hard to buy into a story line where she is not that much stronger than your new PC to begin the game...
What do you think?