personally i'd love to have many of the non-random arcade levels made always-available as replayable stratum 6 optional missions! a lot of these are cool and fun but only rarely accessible through the arcade and not very rewarding.
stratum 5 missions already exist for Jigsaw Valley, Concrete Jungle and Dark City (The Rotting Metropolis is one of my favorite missions to replay for fun sometimes, i love the region), but it'd also be nice to see these for Starlight Cradle(!), Scarlet Fortress, and Aurora Isles - preferably featuring all of their possible floors, and in stratum 6. and yeah, i could also see versions of the boss levels scaled up to tier 3 being added alongside them! (maybe a mission with a couple random floors and then a guaranteed Treasure Vault at the end to make the new content a little more prominent! adjusted for its presence in a mission of course, with the skip elevator removed and probably some reduced treasure boxes so it's not significantly more rewarding than other missions. just to have an accessible treasure mimic fight)
maybe, since OCH is now free and there'll probably never be another DLC expansion, all these optional missions could be held in OCH's shelf of the missions menu? preferably these scaled-up level sets would always be accessible! but yeah, it'd make sense to also have some kind of daily incentive to do specific missions too, maybe one of these optional missions could be highlighted daily or cloned into prestige missions with some kind of extra reward for clearing it once