Whilst it has been years since the release of the Source Engine they haven't yet unveiled a 'next-generation engine' as the Source engine is always updating and is just as relevant in the gaming world today if not more than when it debuted in 2004. It has been aided by the release of the free Source Development Kit (SDK) which include such features as HDR for developers to use in their projects. Hence the reasons stated we have yet to see 'Source Engine 2'.