I am actively looking for a challenge that’s independent of gear, character, and level. These usually wind up being odd combat challenges, but I like them because they’re available to a much broader audience, and if they’re truly challenging yet independent of those variables above, they allow for a lot of creativity. I think @pokapoka had one where the challenge was to kill a raid boss over the longest duration, as in, people were sitting in the ring with these guys for like ten minutes. Arguably, this is harder than blasting them with some broken weapon mechanic before they’re even aware enough to attack, but I thought that was interesting either way.
King of the Hill challenges keep coming to mind as they fit this bill, so I’ll post one of my favorites. Any character, any level, any gear (restrict them as you see fit if you think you’ve got the onions) - fast travel to Highlands: Hyperion Bridge and run down the little rock ridge straight to the platform shown below. Do not stop to kill anything (though for bonus points, you can take a long route around to stir up extra enemies) until you get to that little platform. There, the challenge is to survive whatever Threshers come at you without leaving the platform until they’re all dead, or you are. That’s it, simple (though being pulled off by Rift Threshers is a thing, so heads up). Or rolling a UBA Pyre Thresher… no shortage of potential danger. It’s just a bunch of Threshers, right? I want video, but I’ll settle for pics or at least a story of your character, loadout, build, spawns, etc.
Remember - if you ever touch the ground after starting, challenge failed.