Most MMORPGs add new content on a regular basis in order to justify the monthly fee. City of Heroes for instance has added 2 new archetypes, tons of new missions, new mission types, new areas, new enemies, new costume options, new power sets, etc. They used to also send you an issue of the official City of Heroes comic every month as well, until Devil's Due took over and wanted to sell it in stores.
Anyway, a Diablo MMOG is the next logical step. Heck, they could even call it Diablo 3 and it would probably feel like an actual sequel unlike FF XI.
Starcraft... I'd have to see how its done. WoW is really good, but there are some things that really annoy me. Like level limits on equipment. I understand why its there... but it still doesn't make any sense from an RP standpoint. "Gee, I'm just not skilled enough to put on this hat."
I would love to see a Starcraft single player RPG. Not enough Sci-Fi RPGs, and I think that world would lend itself to some good stories.