Why not go the Disgaea way and put the level cap at 9999? :)
Another option would be that each dungeon has a certain minimum level, but it can be increased.
Example: A dungeon is generated with level 10, meaning that it's suitable for level 10 PC. If a level 5 PC comes in, it will still be a level 10 dungeon.
But if you don't come in until you're level 20, the dungeon will have level 20. If you go in at level 60, the dungeon will have level 60. Of course, once you wander in, the level is locked, so no easy levelling by repeatedly entering the starting dungeon.
This is basically an extension of the small cave system from ADOM.
Another question that pertains to this is how high can you get your attributes. I'd like to see an system without any hard-coded limits, personally, but if your attributes become truly monstrous, it should be somehow reflected in the game. If your strength and toughness grows too much, you should look like a muscular giant regardless on how big you were before. With enough epic toughness and willpower, you laugh in the face of old age and become an immortal of sheer stubborness.
I guess what I'm saying is this: More spread. Have monsters that are easy to kill for Level 1 PC. Have monsters that are challenging for level 200 PC.
And if you get high enough, you explore everything, and there's nothing more to do, build your own tomb, enter a deep sleep, and wake up hundreds of years later when world has completely changed to start exploring it all over again.
Be epic! Be JADEd! :)