This is a suggestion I came up with while looking at my map screen. I have only 100% beaten (That is all badges not all classes) 1 dungeon in the game. . .the Den of Danger.
But honestly it doesn't feel complete. I feel like there should be some great challenge thrown at you when you finish a dungeon so that you can finally say "Okay I have mastered this"
The simple solution is a mission popping up when you get the last badge that requires you to go into the dungeon prepping the vicious token and nothing else.
A viscous purist mission that would maybe give a gold reward (or even better an aesthetic reward of some sort) that would allow you to get that final sense of "beating" that dungeon.
You could make it even more hilarious by removing (or reducing) shop/god/booster spawns but that would probably make several of the dungeons (or all of them?) impossible. Heck as far as I know a lot of dungeons might be impossible with the viscous token unless you have a preps as is. . .
But who needs possible victory when you have certain defeat?
Obviously this is something that people can do anyway on their own. . .but I think it would be cooler with a mission.