I'm probably gonna get a lot of hate for this, but I think vehicles are the next ingredient to making this the perfect fps. Halo CE did vehicles well as it was easy to kill the players inside, why not TF2? It wouldn't break any classes in particular!
Sniper Camper Van = up to 5 inside (1 driver) 3 in back Truck = 6 people, two in front. Any other ideas?