The folks at /v/ from 4chan headed over to Valve to wish Gabe Newell a happy birthday (in a pretty intricate and ingenius manner) but in the fracas some questions were asked, and it looks like Valve could actually be working on a new Source engine.
In the above video Gabe states, in response to a question regarding a new source engine, that they've "been working on new engine stuff for a while" and are apparently "just been waiting for a game to roll it out with".
Someone then asked: "Is it more than just an extension to Source? Like, is it an entirely new engine?"
To which, Newell answered, "yeah".
Gabe also confirmed that, yes, Richochet 2 is in development, leading many folks wondering if it'll be the first game using the new engine.