What would you think of a gym based on a theme rather than a Pokemon type. The Gym Leader would for instance use Pokemon with other similarities than types, e.g. Pokemon that looks like space aliens.
This would make the game difficult when it comes to finding a type to battle the gym with, but other than the difficulty, how would this look? And taken into consideration the difficulty, how would it affect the gameplay?
Example of themes:
Space (use futuristic looking pokemon like robots and aliens)
Mythology (pokemon based on a mythology/similar mythologies)
Animals (cats, dogs etc.)
This thought came to me, because I asked myself, why are Gyms always type based. Can a Gym Leader not have Pokemon of different types or is type a law they must follow? I remember the final gym in RGBY used a mix though, leaning a bit toward ground-type (though I have a theory why this is so).