Im excited but id like to see L4D cross into Portal 2 because zombies and portals are too cool of a combination. I also think that their wont be an Episode 3 and that it will be Half Life 3. My reasons are the following:
6 years between Half Life and Half Life 2, its been 6 years now since Half Life 2.
2 expansion packs were issued for Half Life, 2 episodes were issued for HL2, and Valve has already stated the episodes system did not work as planned, and other than Telltale it seems episodes in general are dead.
The cliffhanger at the end of Ep2 is a perfect set-up for the 3rd Half Life.