Thank god that the people who make these games would not be dumb enough to submit to fanboism rather than logic or common sense and include Revan and the Exile as playable characters.
While a prequel could be possible, the mandalorian wars, we already know the choices the two characters made so it would really make a mockery of the whole point of KOTOR.
A completely new storyline would be the best option, significant cameos mostly are lame, pointless and the lazy, talentless writers way out. A mention of the characters would be worthwhile and interesting though, the inclusion of the exiles deciples as the new leaders of some kind of Jedi order for example.
As mentioned before the reason why Baldurs Gate 2 worked was because you reached level 7 by the end of the first game, then 20 by the end of the second and finaly the cap was removed for the final expansion. The level progression was just plain rediculous in TSL.
Knights of the New republic could be interesting (post ROTJ)
My main hope for KOTOR3 is that they make the game more difficult, in the movies jedis are not godlike, yeh they do kick ass but when seriously outnumbered, and/or outsmarted, they die just like the rest of them. Exceptions being with the very toughest, Yoda, Darth etc. As dumb as Lucas' implimentation of his storylines are in the new films, the darkside does not win through bruteforce dito with luke not single-handedly wiping out the empire.
Ok that was a very longwinded way of saying I hope they add some danger