I guess what I take away from the variety of responses here is that it's unknown what if anything will be added in the way of an endgame, and some pessimism at the possibility. But as most of you must be aware, this game is one hell of an accomplishment as is, it's not easy to build what these guys have built, so I give them a LOT of credit for both competence and vision. As developers, they've surely played this game to death and I'm confident that they are very aware of what it feels like to reach the end and experience that "now what" feeling. This is of course in tension with their principal desire to get the thing shipped, and it sure seems like it's close from my perspective, but then A21 seems to present some pretty big changes, so maybe we're just at the start of a new direction. I don't know, but I'm betting that they have some great things in store for us in the future. It's been a long time since I've been as hooked on a game.
I didn't mean to suggest that I'm level 300, I'm far from it, I think around 160 or so. But I've got all attributes maxed, all of the perks relevant to this character maxed, and a lot of extra goodies thrown in for good measure. But I am at that "now what" point and trying to decide between rolling a new character/map (again) or just playing on to see how zombies progress into late game. Either that, or go find another game and come back later, after A21 perhaps.