Well obviously, the plan to take a sailboat to the Florida Keys didn't work. The military swooped in and bombed the entire dock and its surroundings to a crater. The Survivors are gonna have to find another way to safety. Luckily, a few scribbles on the walls of a nearby safe house tells them of a huge protected zone further down in the south where they'd be able to live out the rest of their days fairly normally. Not like they were willing to give time of day to the military again, but desperate times call for desperate measures. How are they gonna be able to continue such a long ways without Bill's help though?
"Hey folks, miss me?"
The purpose of Bill's sacrifice was to get everyone else to safety, and since that didn't happen, Bill has unfinished business he needs to get taken care of before he can confidently rest in peace and make his way to the afterlife. He'll see to it that Zoey, Louis, and even Francis get to New Orleans if it kills him again (don't ask how that's possible).
An alternate storyline where the original Survivors travel to New Orleans in the place of the Left 4 Dead 2 gang, with Bill's help from beyond the grave. All official L4D2 campaigns are considered part of this canon except for the Passing and Cold Stream. Some quality-of-life mods I like to use are also included in this collection.

I did not make any of the mods included here, I don't have that kind of talent.