Like other, i dont think that the left 4 dead would fare well if its third game was just more of the same, as was Left 4 Dead 2. Now don't get me wrong, I love L4D and will probably keep playing for a long while but it needs to be fleshed out much more. Ideas:
Persistent City Mode/Campaign: New single and multiplayer mode. Allows people to start their own 'saveable' campaign in which the survivors are stranded in a explorable city/island/whatever and must gather enough suplies to leave. Survivors have a main ground base, with a map of the city and stock of starting supplies, instead of 'maps' the city would be semi-explorable(divided into smaller explorable sections? Unlock these sections?) with different goals such as collecting medicine, fuel, weapons and ammo, vehicles(imagine speeding away from the horde in a beat up car only to have it break down, forcing survivors to hold of the horde while and NPC fixes it), or anything else the survivors might need). The Director would spawn infected according to the main goal, location, and team status. Of course, back at home base you could have 'intermissions' between goals in which you could repair/upgrade weapons, vehicles and even your base(as infected could also attack your base).
Since everyone would have their own personal file of this city, teams could be made either through friend invites, City Quickplay(in which you jump into somebody's city), or lobbies with specific City Mode filters so strangers can advance their story together.
More Add-On support: Add-on campaigns add valuable play time to the game. If valve had a weekly/monthly 'featured Add-On' more people would play and create add-ons, increasing variety and therefore game life.
Aside from those two they could have: more weapon variety, more playable survivors(cause who wouldnt wanna play as the Midnight Riders?), more special infected types, and more event variety.
Oh and don't forget a 'kick afk leader' option