Well as for the point of it being isometric view and that making it a poor candidate for mmorpg is unfounded. Look at Ultima Online, literally look at it(http://www.wired.com/news/images/full/uo_f.jpg) it was one of the first really successful mmorpgs and it looks like it works just fine.
The definition of MMO is Massive Mulit Online. Ok well what does that really mean??? I'll tell you. I means there is a persistent game world, even if NO ONE is playing, there are servers running and mobs and npc and everything is just running around waiting for a gamer to log in and experience it.
The only thing that is persistent about Diablo is the hub where you pick out games or chat. Games like Guild Wars are closer to MMO but they are really not. Basically in Guild Wars the online architecture is the same as Diablo, the only upgrade was the hub is now a very small playable area. In a true MMO the "hub" is most of the playable game, with only a select few dungeons and areas instanced for special events.
So could Diablo be made into a mmoActionrpg....sure, could it be made into true mmorpg, no. If you were going to turn it into warcraft you would need healers and tanks.