Then it'll be a new IP. Or a replacement for Source. Valve has relied for so long on Steam and Source, and on three main franchises, after all: Half Life; Team Fortress; Left 4 Dead.
We're unlikely to see an announcement of Portal 3 so soon after the Portal 2 announcement, that would be silly. After L4D2 has only just been released, and there is still a sizeable community for both L4D and L4D2, then L4D3 would be a silly idea. Team Fortress 2 still has a massive player base with new DLC and announcements on a regular basis, so suddenly announcing TF3 wouldn't be the wisest move. And apparently it won't be HL2: Episode 3.
Which means there are only two sensible options for it to still be considered a 'surprise'. One is a new engine to replace Source, more likely than not this would be Source 2.0. That's a very big possibility. Or, it'll be a new IP. Personally I'm leaning more towards the new game engine announcement myself, though a new IP wouldn't surprise me.