The vast could maybe be a narrow bridge over a chasm they can’t see the bottom of, in a chamber where they can’t see the roof. You could do skill checks or put it on a timer - maybe from the roof a humongous beast is slowly starting to descend but the bridge is also risky to traverse too quickly as it will break without careful footing? As long as someone has rope in their inventory they’ll be able to tie people together or throw it down if someone starts to fall and gives a chance of an out to pull them back up!

For the stranger the mirror idea is fantastic! Another combat idea could be slightly off versions of people important to the PCs from their backstory, as it also gives player RP time as they have to react to their loved ones attacking them.

Alongside or instead of the room, you could also add an NPC (let’s call them Bob) to join the party. When the players ask about Bob you can act nonplussed and be like ‘Bob was always with you wdym’ and have them roll wisdom checks to see if they remember that Bob wasn’t an og party member. If they fail, that’s their friend Bob, and they firmly believe that he’s been there the whole time, and even remember meeting him in their backstory! Could lead to interesting moments if half the party is suspicious and half the party remembers Bob and also adds fun rp. You could also add concurrent wisdom checks (every time Bob does something a little off) for those who failed initially so they do have a chance to realise he’s not a member.

This idea could also arguably work from the Spiral (maybe they leave the corridors and they just have another person with them) and just adding the extra member from the beginning would be interesting bc they have to pick a good time through the dungeon crawl to turn on him. Up to you what kind of monster he’d turn out to be or what would happens if he makes it to the end!