In my humble opinion, one of the painful things when leveling up a second (third, forth, etc) class is running the old and same dungeons.

I don't know if it a feasible idea, but I would like to see three things in dungeons (specially in low level dungeons) for more variety:

a) Difficult modifiers: At the beginning of dungeon (or during the "commence check") players can vote in options to increase the difficulty (strong enemies, permanent damage/healing debuff to all players, etc). Each modifier also increase the rewards (more XP and gil from monsters, better loot chests, etc)

b) Special rewards: At the end of dungeon players can earn special rewards (XP, gil, tomes, items) depending of their performance: speed run, 100% mapping, no deaths, 100% monster kill, etc.

c) Random optional challenges (like mini-battlecraft levequests): protect an item or NPC against waves of monsters, kill a stronger version of an normal monster, defeat "x" enemies in "y" time, etc.