A "secret project" to me suggests something other than a current-IP sequel or spin off- so it wouldn't be the next Starcraft or Diablo installments, for example.
If I had to guess, it's going to be another instance of Blizzard trying to get into another genre- they're recently expanded into the CCG, MOBA, and FPS genres after being heavily RTS, Adventure, and MMO focused for a long time. Each of those new projects were held under fairly tight wraps until announced. Who knows what they'd be targeting, though.
I doubt a new project is a new MMO, RTS, or Adventure game, though, as they already have those on lock. As much as we bemoan the death of WoW, MMOs in general are losing popularity these days, and Blizzard wouldn't want to compete with one of their still-successful flagship titles.