Well, there could of course be very specialized gyms that would do more to aid whatever theme of Pokémon (if any) the gym leader might have. I think the usual type variants would be fine -- though have you ever thought of having a mono-color GL? Like.. there could by a gym full of grass and vegetation, and the GL would have a team of all green Pokémon that would have a definite advantage.

Or, I dunno, something like that.

As I said before, specialized gym areas would be fun, to test your skill in battling in awkward places. :/ I dunno, just brainstorming ideas.