After reading some old postings about Sword of Glass on CRPG Addict's blog, an idea came to me.
A RPG that you play as a sentient piece of equipment (probably sword or other weapon, or perhaps could it be customizable?), who is taking over their wearer in the quest of getting deep enough in the dungeon to <do something quest-related or whatever>.
You are getting experience, as well as your bearer/wearer. Your XP is permanent, but the bearer/wearer can die and drop you in the middle of the trek through the dungeon, and if it happens, you need to wait for some other adventurer to come along and pick you up. You can steer bearer/wearer to some degree (perhaps most of the way, to prevent the game from being too boring), but he/she might oppose you if you are trying to push him/her to do something risky/stupid (after all, all you can lose is some time).
Perhaps add in some time limit, so every time you lose a bearer/wearer you waste some of time waiting for a new one on the floor, and you need to be more careful around them? Also, as the time passes, topmost levels of the dungeon become cleared by adventurers (perhaps mostly by you and your bearer?), and you can go deeper more easily (as in, with easier trip in; the level X would stay equally difficult through the game, unless it was cleared). If you are lost N level below the last cleared level, you need to wait longer till someone gets so deep and finds you.
Or maybe you could be picked up by some orc/demon/drow in the dungeon? And you would be wielded alternately by good guys (and lost in the dungeon), and then bad guys (found in the dungeon, lost in the overworld during the ride), and then good guys again (found in the loot after a failed orc raid)? And you would need to choose the side you want to win, and try to keep your bearer alive as long as possible when you are on the 'right' side, and kill him as soon as possible on the 'wrong' side (but keep in mind he/she will resist you trying to kill them right away!).
Seems like an interesting twist on more classical RPGs.