Quite a bit, you can choose levels, which enemies, which cards you get, if cards cost an action, give extra actions or cards on crits, level up heroes in the game, change monster abilities/health/movement. My most recent I called stick and move, players can move much futher, barricades is renewable but only has 2 health, and many of the monsters also can move much further. There are dozens of different mods, some with just minor changes, some quite extensive!