Kinda nonsensically, I'm gonna start with why this doesn't have to be a thing: there are dyes for the hardcodes, a title for one dungeon, and another title for getting a large number of the achievements.
However
I think it would be really cool if we got one achievement for being truly undaunted. it doesn't have to give anything, but just an additional "you've done all of the base game's 4 man, look at this little picture and non-faded text" kinda thing.