There's still a lot of story to tell after DII, in fact, DII:LoD had an open ending...
I could be wrong with the next thing, but the bosses you kill in the game are the living bodies, and the soulstones contain their soul, therefor if you destroy the soulstone, their souls fade away. This was the mistake at the ending of DI, and diablo's soul manifested itself in the adventurer that killed him. This mistake has been corrected in DII where both Mephisto's and Diablo's soulstones are destroyed. Baal's soulstone however has, to my knowledge, not been destroyed yet (can't fully remember it, but baal took it from the feller who pulled it out of Tal Rasha in the movie between actII and actIII (you see this in the end of DII (not LoD)). The only Prime Evil that can return now, is Baal. Also, if people read the manual that came with the DII (or LoD) game, then you can see that there is a map in there. You, as adventurer, have been everywhere on that map, except on the Amazon Islands. In short, I think that Baal has a possibility in coming back (him being the youngest (and maybe even strongest) of the three brothers) and maybe that will happen through Tyrael, in exactly the same way how Diablo came back.
But still, enough story to make a third game with the Horadrim and all too
Just my opinion
