Once word got out that Valve had cancelled its Portal 2 event and is opting to announce a "big surprise" in its place, rumour had started stirring up regarding what that surprise may be. One of the biggest speculations of the surprise is an announcement of the third chapter of the Half Life 2 episodes.
Granted Half Life 2: Episode 3 would be a big surprise, but according to Valve boss, Gabe Newell, this won't be what the big surprise is. Newell has also announced that the teaser image that made its rounds on the internet last week is, in fact, a fake.
So this new announcement is still open as to what it could be. Though the option for Half Life 3 wasn't ruled out quite yet, there is still the possibility of an entirely new IP or even a new Team Fortress. Whatever it is, you can expect to find out this next week at E3.