Team Fortress 2 for Left 4 Dead (or are straddling the two), then you'll want to start digging through this comprehensive new piece from The Escapist for hints about the cartoonish shooter's future. After a quick perusal, we've already spotted a tease for a "very different" new game mode (coming soon!) and discussion of a possible 10th class.
Speaking of L4D, we'd like to make our bid for the 10th class: The "Louis." His special skills include 1. being Louis and 2. shouting about pipe bombs. Get it done, Valve.