Coming out shortly for Left 4 Dead 2 is the new DLC campaign "The Passing"

"The Passing," the first add-on for L4D2 brings the original Left 4 Dead Survivors down south for a meeting with the L4D2 Survivors, while delivering new single-player, multiplayer and co-operative gameplay for the PC and Xbox 360. 
It  takes place just after the Dead Center campaign of L4D2 and is set in a small town in rural Georgia, players assume the role of the L4D2 Survivors as they meet up with the L4D1 characters.

In addition to the new co-operative campaign and associated storyline, The Passing includes new arenas for Survival, Versus and Scavenge mode and introduces a new co-operative challenge mode of play. It will also include a new "uncommon common" zombie class, melee weapon, and firearm namely:

A Wicked Witch Bride

A M60

New Game Modes

A Golf Club

And Introducing The Fallen Survivor!
Only 3 of the orginal survivors will make it to rural Georgia, 1 has passed on but which one is it.  Will it be Bill, the grumpy yet plucky geriatric, Louis in the time honoured movie tradition of the black guy always dies first or Francis the misfit biker who tells them all he really likes them and to be honest, I don't think Zoe will kick the bucket as she is the only woman in the group.
According to Valve writer Chet Faliszek L4D1 players will get to find out first-hand.  After The Passing is released, new DLC will also be released for L4D1 that actually has gamers play through the events that ultimately claims the life of the doomed character -- and one of the players has to literally make the sacrifice within the game.

Release Date to be announced