I'd be surprised about L4D3, that's for sure.
Hmm...Episode 3 would not be a surprise at all, unless they announce it to be released immediately..(or "it's a not Episode 3, it's
Half Life 3", but I doubt that very, very much)
My money is on they'll announce the project icefrog's doing for them.
They started working (or at least announced working) in october 09.
In a recent Q&A he said:
Q: When will we hear more about you and Valve? Some people say it will take 2-3 years, can you give us any clues? (from Zeyall)
A: It certainly won't take that long. There are a lot of very talented people here at Valve that are working hard on it every day to ensure high quality in a timely manner. We'll be making an announcement soon actually,[...]