I'm not fooling anyone so i won't try, i saw this in another game and it seems like a great idea.
Give old dungeons achievements like speedrun, nodeath or speedrun+nodeath. And add fashion rewards for that.
As an example:
Clear cradle with no deaths in your group - 1 achievement with a title, i don't know, "HAMSTER godslayer"
Clear cradle with no deaths in your group within 15minutes and minimum IL enabled - 1 achievement with a fashion piece, a robe, boots, anything.
Same thing for every dungeon and trial. I would imagine it's not a large amount of work and it would give veteran players and completionists something to work towards, heck, it would give people who are into fashion something to work towards as long as the fashion rewards don't look like, ehm, out of season april fowls jokes (see what i did there?)
As for how to set the timer for the achievement i'm very sure you guys have alot of systems in place to see the fastest runs, the average time spent in an instance etc. And if not i'm also very sure the community would gladly help.
Tomm and Zariel both have such achievements and it was great to work towards them, also i'd say it greatly extends the life of certain content.