Idea I... actually had a while ago but started thinking of again

Massive Valve crossover game. Like, it'll be this mission-based fps with rpg and puzzle elements. There'll be the main mode, which has you going to levels which combine elements from different games (e.g. a war starts between terrorists in counter-strike and the day of defeat army), and there'll be a multi-player mode which consists of the heroes each working together cooperatively and using their character's individual talents to beat a level (e.g. someone playing as TF2 characters have to fight off some L4D zombies while some Portal characters have to get a cube somewhere to make a door open)

Games that need some representin'

Half Life

Portal

Team Fortress

Counter Strike

Left 4 Dead

Day of Defeat

Alien Swarm

Defense of the Ancients