In my opinion Blizzard should be brainstorming the next big thing. However they currently have 3 golden child franchises. Warcraft, Diablo, and Starcraft. It would be a risk, but one they should take to expand into a fourth. However I do not feel it should be in the MMO market. In the past there were very few mmo games and the audience was limited. UO, Everquest were really the big ones with about a half dozen that came later that could not touch EQ's numbers. Along comes WoW in an over saturated mmo market all fighting over the same 3-4 million players and it does what no other game did before and that was expand the audience by leaps and bounds.

I think Blizzard is better off makeing wow bigger and better and do what it takes to bring in new customers than to launch a second mmo that will only compete with the former. I would rather see the resources spent on WoW to get content out faster and updateing vanila wow to the current storylines. I think if they launched a new campaign targeting new players like a vanilla server that can only be accessed by these enabled accounts followed by a free character transfer to an established server at 60. Thus they dont have to compete or feel 5 years behind everyone else that plays.

For current customers, they could launch a pair of progression servers with some world event unlocks like AQ to get to BC and Northrend.