I just think KOTOR 3 will have a lot do to with finding the True Sith, and obviously be connected to both games.
KOTOR 1 ends as any normal action game (although it isn't) with the hero having an ending best suited to their playing style. No questions asked, the game ends, galaxy saved or not. Like Plano said, it is a very "open" ending.
In KOTOR 2 however, many questions were raised. KOTOR 2 ends with Kreia telling the Exile that she must go to these unknown regions, leaving behind her loved ones, just as Revan did and search for Revan. Carth at the end of the game, hints of some future story of the exile finding Revan, and reporting the Carth is following orders, just like Canderous, and perhaps Bastila too.
KOTOR 1 and KOTOR 2 all revolved around a search for something, be it the Star Forge or your revenge / redemption (through the hunt of the jedi masters) . KOTOR 3 will probably revolve around that concept, the goal of your search being either finding Revan or the Exile, or a trace of them, their path, in order to defeat the True Sith (or join them, DS ending ), or either something related to the true Sith.
Apart from that, I just think it would be awesome to FINALLY wage war, either lead it or take part in it. I was excited when I first played the Onderon Civil War since I thought that I would be fighting against hindreds of soldiers, not a maximum of 50...
A plot my friends and I were thinking about is that you are a new Jedi, or the Exile / Bastila's padawan, and you are recruiting an army. I just feel that KOTOR 2's LS plot of finding the Jedi Masters and reuniting them under one cause ended abruptly, either because the Exile killed them all, or because the old witch did. Many characters come back in play, such as Canderous (having reunited the Mandalorians), Carth (just like Revan asked, he has strenghtened the Republic) and of course the Jedi (you find any remnannts of them, and you unify them).
This war however will be at the end, since your PC, having completed the mission the Council gave him/her of finding Revan and the Exile, returns, either with the Exile, Revan or both / none, with the sole purpose of preparing the galaxy against the fury of the true Sith. You could say that Revan has awakened them, and they are aware that their plot to exploit the Republic's weakened state has been revealed.
To sum it all up, your task is first to build Revan's army, one that he will command, along with the Exile and yourself at his side, and together you shall launch one hell of an assault. Of course, now this just seems like Age of Empires II....
Yet the war could be external. You do not take part it in, and you continue the story's plot, doing something "behind the scenes" .
Some titles that could be assorted to this sort of plot:
KOTOR III : Shadows of the Force
KOTOR III : The Sith Awakening