A while ago, I was talking to a few folks and saying that the delay for the next Half-Life
is probably due to Valve Software wanting to put it out in a brand new engine. Seeing as Source is pretty damn old (but still very capable), I wouldn't be surprised if they are wanting to do coming out party for the new engine with the next game.
Well, that case may be as someone's found references
to a Source 2 in the code for the Source Filmmaker program. There are also references to a Hybrid engine, so that could mean an intermediary engine. Either way, it's exciting to see that there will be a new engine coming and hopefully it won't be too long before we see it in action.