I would be a Bug type Gym Leader. I like bugs and it would be fun. I would be the last leader you face before the Pokemon League. My Gym would require you to complete a series of battles facing every bug Pokemon(non legendary) to date. The gym will be a big cocoon puzzle and I would be in the back of the gym awaiting your challenge.
Scolipede lv 55:
Yanmega lv 54:
Leavanny lv 53:
Heracross lv 53:
Volcarona Lv 56(I know it is underleveled, but that is legal for gym leaders):
If you beat me you will receive the Cocoon Badge and receive the newly invented TM 101- Quiver Dance.
Also as a reward for defeating the strongest leader in the region you also will receive an egg from my prized Volcarona