This is very similar to one of the updates applied to the now long gone City of Heroes MMORPG. Whilst a great idea at the outset it soon became seen as one of the quickest ways to level XP and so the game as a whole suffered.

To be honest though I think there are far more than enough dungeons in FFXIV as it is right now for people to remember what to do and when in order to avoid wipes. Also the player created content would have to be monitored somehow and that all costs - adding in an editor would probably be far, far too enpensive.