I actually did an impromptu Archipelago run this weekend. I played Pseudoregalia, which at first I kind of dismissed as a furry platformer with way too much open space, but it grew on me as I played it. The randomized powerups worked really well, as the level design encourages different approaches to each room.
Last year I tried
Celeste (Open World), which was also really good. You unlock interactable objects like moving platforms and feathers as you progress, but you can often skip them with some clever platforming. It's a good spin on things, and the strawberries act as "checks" so you're also contributing to others' games.
Doing an Archipelago round with 50+ people sounds bananas. It's a super cool program, though the setup is still pretty difficult for a lot of games. I imagine if they can streamline the process, they'd really have a hit.