What about the NonMonotype Challenge? (Sorry about the name, I couldn't think of anything good)
This is the opposite of a Monotype. For example, if you did a Fire Non Monotype, you can NOT use Fire types. You can also do more than on type eg for a FireWaterGrass Non Monotype you cannot use Fire,Water or Grass types.
I know it sounds meh-ish.
I think that would work well as a additional rule in a challenge, but it isn't enough to stand alone, imo. Because if you just leave it at that, it's just like a relaxed version of a monotype.
Also, I forgot to mention, with my trainer class challenge, you would play through it like that character would, so, just for example you're a cave explorer, instead of just grinding anywhere, you'd do all your grinding in caves, and you'd thoroughly explore them. If you were a gardener, you'd use berries instead of other items, and if you were in hg/ss, you'd collect the apricorns everyday. I think for ease, these things would be called restrictions
I am in two minds about it though, it would be awesome if you totally created a class yourself, and you'd be able to make it as easy or as hard as you like, but if you had someone else impose restrictions it could be more exciting/ harder/ something you wouldn't usually do. I suppose if this does come to fruition, when signing up, the person could specify if they wanted another person to come up with restrictions.
I suppose it could be used as a kind of role play. I've always been jealous of the other trainers, how they can specialize, and I think it would be awesome if you could become a proper pokemon breeder in the game, or something like that, but I think this would the closest we'd get to that.
People could really get detailed too if they want, they could decide that the trainer class might not be into breeding ( for example, trainer class macho man) so they wouldn't breed.
I want to test it out, but I can only find my heartgold atm, and I don't want to lose my good pokes...