First I'd like to state that this is pure fiction and an idea I had to merge Diablo into the Worlds of Warcraft realms.
While WoW is deeper than the Diablo series, it could certainly fit. There would be instanced dungeons, with each one different but with a Diablo feel to them. There could also be dungeons that are randomly generated the same way Diablo levels were with others being set in design. I don't recommend too many random dungeons though, maybe 5 with 10 dungeons that are set, all of them would be instanced though.
I like the way Everquest's Lost Dungeons of Norrath handled getting missions and people having to be within certain level ranges of eachother. I'm not certain how it would work between the two factions of the Horde and Alliance though. Either each side would have their own entrances into such dungeons, and/or they would be in the middle of contested lands like the battlefields. The quests would then need to be given out by a neutral being, or race of creatures. Angels would probably fit the theme. As far as the Horde the Angels could overlook their sinning ways as they are desperate and will take any help they can get at fighting ****'s armies.
Just some ideas I wanted to throw out, Anyone else think this would be cool? or have ideas they would like to add?