Destructoid is running a rumor they've heard about a sequel to Left 4 Dead with four different characters, similar (yet different) areas, and a new boss infected. They liken it to Ada Wong's side missions in Resident Evil 4 that had the second character running through the same areas in differing orders.
Users at Neogaf think that it doesn't seem like Valve's style to release this sort of rehashed levels with a couple of new characters. And I agree. I would think that Valve would be releasing new campaigns for their existing Left 4 Dead title.
As much as I'd like to see some Left 4 Dead news at E3, I'd much prefer Valve showed off something brand new that no one has seen before, or something on Half-Life 2: Episode 3.