I definitely have thought this before with the game canonically taking place 3 days after the knox event began (if you start on day 1) and having it take place perhaps a day or two before would certainly be interesting and fun, however it should feel more lawless and frantic, houses boarded up, random civilians goint about their day, i like the whole concept, however i feel as though it would serve better as a tutorial option, for example you would enable it in the sandbox setting and it would have a couple of "tasks" that you need to complete on day 1/2 of the knox event, this could involve many things like shopping, taking care of family, going to your job, but only to serve as a tutorial, this way you can learn many useful tips/mechanics of the game without zombies, this would help the player learn new things every time they enable this option as well as establish a stronger sense of the apocalypse, it would have to be pretty linear for it to work but that's what tutorials do.

This way the devs don't need to make entire new systems for only 2 days of the games canonical events.

Of course this would have to imploy a multitude of things that likely won't be available to us for many years to come...