So, you have this Reverse Tower of Power. In theory, the levels would be infinite, but would become impossible or just extremely tedious or hilarious.
So you'd start out in some random square like room, there's be a door in the middle, leading to level one. It has some pillars, waist high walls, etc. Some run of the mill goodsprings-caliber enemies. Like 5, maybe. Kill them. You get a 50% 9mm pistol. Yay.
Then you go to levels 2-5, no big deal, average stuff, rewards get better. This is where the game gives you a chance to return to the surface. A checkpoint. Every five levels until 20 would do this, then 10 more to 30, then 20 to fifty. You can leave if the fight is too much, but, it restarts from the last check point, depending on your luck, other bad things. Like 1980s sitcom laughter at your pansy ass for not preparing for 20 levels of hell. And revoking items won after a checkpoint.
So when you get to level ten, bad mofo creatures, Super Mutants, and boss level Fiends. Then it's the FLASH BACK ROUNDS at level 15-20 where you fight Talon Company Elves, Paradise Falls Gremlins, and FO3 Enclave (who ironically accompany "Col. Anti-climix" who dies if you point your gun at him or breath to hard in his direction.
LEVEL 20: DLC BOOGLALOO: Chinese Dragoons with CSAs and stealth powers, SUPER REAVER 64s, Overlords, Aliens with Alien weapons, and EXPLODING RADIO GAS
LEVEL 30: Dancing (with death) with the stars. There's a resource that makes celebs playable, so you could fight Patrick Bateman, Jack Bauer, Dirty Harry, Bradgelina, and for Level 39: Chuck Norris
40: COMMONWEALTH REPLICATES, BY THE DROVES. Uses standard npc races.
And it hurts.