Quote Originally Posted by Kevvaelli
How about just a general theme? Example, in Dark Sage's "Pokemonese" fanfiction, the gyms have a general non-type theme, such as reptile-esque Pokemon or all-female pokemon.
While reading the posts I also thought about something like that. Gyms' pokemon could depend on climate, humanoid or animal form, pokemon size, mythology or even personality or other stereotypes, I don't know.

On the other hand, I do believe they should have a TM that differs them from everyone else. After all, they are gym leaders right? TMs previously approved in the tower of course.