I would love to have themed typed gyms. But it won't increase difficulty, just fresh.
The types I am expecting though are Water, Fire, Flying, Steel, Ghost, Ground, Rock, and Dark. I think this would be the best time to have a Dark Gym leader with the Moon being the theme. And to be, Fire, Water and Flying are given due to it being in Hawaii.
I would love if they used Double/Trimple Battles. Rotation I don't think should be used, because those are just time consuming and hardly make a difference. However, Reversal battle would make a gym hard for veterans who are perfect with types weaknesses. I actually struggled with Reversal battles and found it hard to learn them.