So what will Valve announce at E3?

Absolutely nothing.

According to G4TV, Gabe Newell has sent out an e-mail. The email in question is brief and to the point saying that Valve is not announcing anything new at E3 this year.

We are not announcing Half-Life 3 or Portal 3 or Left 4 Dead 3. We are going to be showing stuff everyone already knows about.

Basically stuff like CS: Go, Dota 2 and what not. For those of you who were hopeful that Valve was going to announce Half Life 3 hopefully something solid will come out about the game soon.

Newell was thought to have talked about Half Life 3 in an interview with the Seven Day Cool Down podcast. A question about Ricochet 2’s announcement, Ricochet being a mod in Half-Life, was brought up and Gabe answered in a matter of fact manner that he wants something solid on Ricochet 2 before announcing it. Ricochet 2 being the obscure reference to Half Life 3 presumably.

Source: G4TV