Valve's Source engine has been running most of the company's games for around a decade now, but despite some wizardry being performed on games like Left 4 Dead 2 and Portal 2, it's starting to show its age. Which is probably why the company is starting to ramp up the number of times it mentions its Source successor in public.
Back in August, some notes were found in code referencing "Source 2", then last week, during a fan visit to Valve's HQ, boss Gabe Newell said that the company was working on a brand new engine, not simply an upgrade to the existing Source Engine (which has been updated several times since Half-Life 2's release as it is).
I won't even bother asking what your thoughts would be on what game could possibly debut a new Valve engine.
Gabe Newell Confirms That A Next-Gen Valve Game Engine Is In Development [Lambda Generation]