
Another related idea: I think it would be awesome if characters could enter the infinite dungeon (at an appropriate level) after beating the game. "Sorry, Melinda, I'd rather hunt down a god and a sher'tul godslayer... goodbye forever!" Doing it this way, though, I think the player level should still be capped at 50. This primarily lets a character go out in glory:
"Do you think your winning character is invincible? If so, then see how deep you can go in the infinite dungeon! (you will die eventually)"