Overwatch will be big, no doubt about that but in terms of an announcement? What is there to announce?
SC2 will has it's expansion soon so we might get a release date. Is it huge though? Not really.
Hearthstone had it's expansion announced, almost as if they don't want it to be overshadowed by a different announcement.
Heroes of the Storm has it's Eternal Conflic thingy, is it HUGE? I have no idea. (Not following it a whole lot)
Diablo 3 might have some spotlight, however it won't be playable this year so I doubt there will be big news. Also 2.3 will release soon.
WoW had 6.2 not to long ago. Would that justify it beeing playble though? I mean they could have handled it like Diablo and not make it playable however it will be.
A new IP/Game it could happen, like the past has shown blizzard announced smaller titles outside of blizzcon.
tl;dr:
It will either be WoW related (X6 or 6.3) or a new smaller title.