The rumors were true and there will, in fact, be a Source Engine 2 according to a recent video by 4chan.org. In the video 4chan’s game community, /v/, visited Gabe Newell to wish him birthday and take a tour of the valve headquarters.
When asked about plans for a new Source 2 engine, he replied ““We’ve been working on new engine stuff for a while, we’ve just been waiting for a game to roll it out with.”
Apparently, it won’t be a simple remake to the older engine, the engine will be totally new. You can check out the 50+ minutes video here