Given the success of both the original Source Engine and current middleware leader, Unreal Engine 3 (soon to be 4), it was a bit of a no-brainer Valve would eventually roll out something akin to a new Source Engine.
Comments made by Valve's Chet Faliszek last year suggest the team prefer to iteratively add to their current codebase, but he also loosely adds that there's likely going to be a Source 2 at "some point". That was muttered roughly a year ago, and more evidence pointing to work on a Source Engine 2 appears to have emerged after a recent visit out to Valve HQ in Seattle from 4Chan’s ever-active /v/ board (so obviously take this with a grain of salt), where Valve boss, Gabe Newell, allegedly offers tantalising lines towards a new engine.
"We’ve been working on new engine stuff for a while. We’ve just been waiting for a game to roll it out with," Gabe is reported to have said (thanks VG247
The reason for the sketchiness of Gabe's alleged confirmation, is because the visit is captured via a mobile and has terrible audio. The uploader has added captions to it, but the audio isn't clear enough for a 100% nod.
Watch it on YouTube here
to make up your own mind, but if it is true and the team are just waiting on a product to "roll it out with", might he suggest HALF-LIFE FRICKING 3!!! *Ahem*, back to work then.