if anyone can think of anything that you want to be able to do only after beat the main quest, and the other factions, and/or and you reach a very high level (like 50), then post it here. I might make a poll if we come up with some good ideas.
my (rather half-baked) ideas are:
-a gargantuan dungeon (like 50+ floors) full of incredibly dangerous monsters, and no places to safely rest (the end would be a good place for a super-hard "boss" to reside.)
-some kind survival-arena-mode, where you take on wave after wave of enemies with limited items and no rest.
-similar to my other idea, a time-trial arena mode. Limited time to beat X number of [insert enemy here]
^the latter two could be in the game from the get-go, but if they catered to the super powerful players, and gave them a challenge and maybe something to work for, then i think it would extend the amount of time i play the game.
reconstruct or deconstruct my ideas if you want, but try to think of other ones too.