TBH I think those holding out for Diablo are in for a long wait. As has already been mentioned, WoW's success is in attracting new players to both the world and the genre. They are likely to capitalise on this for some time. Doing another MMORPG in a similar genre is going to reduce the income / effort for them, changing the theme (SC style) will perhaps net some new players but will also dilute the current WoW player base. I would put money on it being a persistent environment but with a very different play style, perhaps an MMORTS?