Another who prefers team constraints. Done a few monotypes and a scramble or two. Also Leaf Green with first-stage Pokemon only, and SoulSilver with Pokemon that don't evolve only (eg Lapras, Girafarig). I'm planning on doing a monocolour run in future, that should be neat.
I tried Nuzlockes a couple of times, didn't like them. Without a level cap it just turns into grind like crazy so you don't die, while with a level cap it becomes grind like crazy to replace the team members that just died!
I think I'd like to do a low-level run, though I'm not sure Pokemon's the most amenable game to that.