I would really appreciate being able to import a bored 1000-day survivor from the mainline game and subject him to ever-increasing swarms of enemies in a mansion, hospital or mall.
Seriously, add a mall into the list of possible locations.
Could also define a few “boss” enemies that don’t make sense in the normal game but would be cool to have in late-game defense mode. Giant armored robots, otherworldly behemoths, zombie titans…
Oh, and does the generator heal its damage eventually?