A lot of the people responsible for Diablo I & II have since left Blizzard, in fact several of them did Hellgate London. Blizzard has shown some renewed interest in StarCraft with StarCraft II, and this development tract looks very similar to that of Warcraft's. That is taking a 2D RTS to 3D then MMO.
All we know is that Blizzard is indeed working on another MMO.
Gamasutra - Blizzard Confirms 'Unannounced Next-Gen MMO'