Flushed out by a community site, the Source Engine 2 has been recently mentioned by Valve team which is in developement.
According to some files found by the site ValveTime, the developer is working on a mysterious project named "Source Engine 2". In all likelihood, the famous project would be a new version or a "next gen" engine from Valve, directly from the Source Filmmaker (a freely distributed version of the engine and to make animated films). While waiting to learn more about this Source Engine 2 project, the developer believes that "without boasting," it will be even better than the current engine.
It thus becomes quite likely to be seen a formalized development of this new engine, probably at the time of the announcement of new titles based on the Source Engine 2.