Either Episode 3, an entirely new game, or an entirely new engine (that would be a complete shock and it would at best be an announcement that they're gonna start work on one, but with Source being as good as it is considering their size when they made it and the date it was released, I would LOVE to see what they could do with a brand new engine in the future).
The L4D The Passing DLC is something I DO want, but it's not nearly as big news as a new game or anything.
Personally I think the surprise is LIKELY to be (and I wouldn't mind at all if I was right) that they're further on with Portal 2 than previously expected, perhaps the announcement of a playable demo or something of the sort, shame it's too late to add that to your poll options though, and I doubt a lot of people would vote for it anyway.
