Two questions: Do you think we will meet Mephisto and Baal in Diablo 3? Do you want to meet them in Diablo 3?
Seeing how they can bring Diablo back, even though we allready killed him, they shouldn't have any problems with these two if they wish to include them in the game. I am torn. Diablo wouldn't feel the same without them, but I guess it would be kind of repetitive with them.
Baal is easy. His stone was never actually destroyed, unless the Worldstone counts (which, for the sake of bringing him back, it doesn't have to).
Meph is a little bit more difficult. Not only was he killed, but his stone was smashed. However, if they can find a viable way to bring Diablo back then Meph would work as well, and likely be back by default.
Diablo without Diablo isn't Diablo. Diablo without Mephisto IS Diablo.
IMO, new game means new act Bosses. Since Meph can't really change, it'd be boring. The whole point is waiting and fighting your way to see the bigass Boss.
Well unless they wanna keep the franchise going forever they have to come up with a way to get rid of him or make it impossible to access sanctuary. Mind u its not a problem for me if they keep making diablo games until i get old