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I actually think it would be cool if players made their own dungeons and the reward wasn't XP or Gold, but FUN and gameplay.
It could actually be an entire mini-game, where players get to design their own dungeons but as they level up in the real game, they get items such as different terrain pieces, collect monsters and NPC's to use, etc.
I think this is a lot better of a choice than games like DDO or WoW where the dungeons are entirely instanced anyways. Honestly... even the better dungeons of WoW, I would still prefer player-made dungeons. Even my favorites aren't *that* amazing. DDO had some amazing dungeons, so that's different. However, replayability in BOTH of these games was severely lacking. Something user-generated content easily solved.