There could even be an entire Survival Exploration element to the crater that would require you to stock up on the supplies needed to survive it, from things like Rad X, antivemons, ammo and special armors to even needing special oxygen masks like in the Metro games. Maybe have some underground facility under the Crater, turn the metro tunnels into an optional dungeon with much more going on for it, maybe even have a cave in that connects to natural underground tunnels, mines and such, where you would find deadlier critters and such, maybe even find other scavengers, adventurers and just opportunists in your excursions. You could even find safe spots or alternate access routes where you could set camp to continue exploration and being able to restock without having to restart is every time. There is a lot that can be done with it.