Hellgate: london seems to be the "spiritual" continuation of the series. Alot of th people working on it worked on some diable game, and the dynamics (from my understanding) are supposed to be the same, plus third and first person view.
Diablo III will probably borrow from Dungeon Sieges view (overhead and sweepable instead of being fixed).
I don't know though... I think Diablo is either dead, or going to get an overhaul so outragious that it will barely resemble the other 2 games. Would have made a great MMO probably... but that would either dig into WoWs monopoly of players (which I'm kinda hoping Warhammer Online does), or render the game so void of players that it wouldn't even be worth the cost.
The future of diablo seems rather disappointing to me... oh well. Whatcha going to do about it?