Awesome tag man. Much deserved.
Cheers.
That's a good question and I can only guess. I think there's definitely a chance Source 2/L4D3 will be revealed this year, Valve have been referencing Source 2 at Steam Dev Days a couple of times. It should be in an advanced state now. But the thing with Valve is they don't seem to like overlapping/parallel communication. Like this month it's all about TI, next month CSGO will have a major tournament and they probably want to talk about Steam Universe stuff again. So I think Fall could be the earliest, I'm just not sure how much sense that would make as Fall is very busy with game releases. Do we usually get lot of reveals during that period?
If we assume L4D3 is being prepped for a Q4 2015 release I'd say the earliest we'll see official releases of media will be March/April next year. I just hope there's another reveal ARG as the Portal 2 one was fun to follow.