In the past, the first game that actually featured Source was Counter Strike Source. Half Life 2 came out after CS: Source. Base on the recent DOTA 2 fan's photography of the Valve's computer log, we knew Source 2 and L4D3 are in progress which could mean, the first Source 2 game will be L4D3. I am fairly certain Valve is working on HL3 and I am betting the game will be much larger, perhaps more like an RPG since HL2 format is no longer sufficient and generally, HL2 is very rich that can support RPG.