Ground Gym would be awesome, yet threatening.
Ice Gym has it's advantages.
Dragon Gym proves challenging in SS, so it probably still will.
Rock Gym....too easy.
Bug Gym is alright, but fairly easy.
Psychic Gym can prove difficult.
Water Gym isn't hard, but Nintendo uses that alot.
Fighting Gym is a good choice.
Poison Gym would be awesome.
Grass Gym....also too easy
Normal Gym has it's difficulties. Especially the high HP advantage.
Ghost Gym can be challenging.
Dark Gym as well.
Steel Gym can be improved...
Electric Gym has its difficulties, but have been used alot.
Mixed Gym may be a new level in gym types. Maybe a gym with restrictions, just to make it different...
I can hope Gen V will make the gym's better than some of the previous games.