Eh, any non-cosmetic reward makes you more powerful, and the game is pretty much biased towards the player as long as you improve until you're good enough to take on the hardest dungeons. Any cosmetic reward boils down to a variation on the current completion class badge ratings.
I had an idea ages ago that tied the idea of completionism with the storylines for each direction. If all of the dungeons started at a high difficulty but got their difficulty reduces by completing the lesser dungeons with more classes it would be interesting. If the devs wanted to keep the original structure of the game, they could apply it only to the vicious dungeons - make them damn near impossible to begin with, but make completing the regular dungeons reduce their difficulty. For example, completing every directional with all guild classes would get the monster difficulty down to the current vicious, completing with all church classes would give you your altars/piety gain back, completing with thieves would give you gold/shops and completing with explorers would give you subdungeons. It's just an idea, god knows if it's feasable, and unlocking the monster classes would be an obvious pain.
But other than that, I can only think of the locker/veto system to give more meaning to gold once it starts to pile up (not going there).