The rumour of a possible sequel to Left 4 Dead has gotten me thinking about the actual likely-hood of seeing a follow-up to Valve's top quality co-op zombie shooter. Does it need a sequel? The straight answer is no. It's stating the obvious to say that Left 4 Dead (and Team Fortress 2) have been built with long teem expansion in mind. The core gameplay is so spot on, that there's nothing really obvious that could be improved upon in a sequel, at least in the usual terms that we think of in a sequel. Normally when a sequel comes out, it does a handful of things: it either adds (or fixes) new mechanics and abilities or it continues a story that the previous game(s) either didn't finish or needed to be built upon. A sequel is a something that gives fans more of what they want or make wholesale improvements. It's also a way of keeping people playing the franchise and cements it in gamers minds as an ever-present presence in the gaming world.
In this sense, this is where the downloadable campaigns would fit in; these can be almost endlessly pumped out and would be welcomed by fans as long as Valve kept up the quality. There can be endless tweaks to the gameplay (such as the fatigue bar for melee attacks in Versus mode), plus new modes like Survival mode. What about new characters? Maybe a new update would give us four new characters that you could select, giving you a truly different experience every time you played. New special infected? Possibly, but Survival mode has highlighted how good the special infected work. The Boomers blind you and summon the horde; the Smokers help isolate survivors and stretch the group; the Hunters pick off stragglers and immobilise players. It's hard to see what role a new special infected zombie would bring to the game, but nonetheless a new special infected would be welcome. There's Left 4 Dead players out there who are pro's at this point. They've completed all campaigns on expert and gotten Gold medals on all Survival levels. They know how to deal with the common infected and the special infected without too much trouble: a new special infected would spice things up for sure.
I think in our cynicism it's easy to see EA trying to push for a sequel to a critically and commercially successful game, but I believe we won't see a sequel to Left 4 Dead at E3 this year. Valve call the shots and don't believe (but could be wrong) that they would push announce a sequel just over six months from the original. Next year or the year after? Very possibly: if Valve come up with a completely new graphics engine, then I'd think it would be more likely. I think the more likely (but still not as certain) announcement would be a big update: new campaigns, new modes and such. But a full blown sequel? Not here, not now.