And since it is going to be hard to become the leader there should be rewards,mostly aesthetic ones like GYMLEADER before your name in the chat,maybe legendaries?
I had a similliar idea.The way your idea works now would make leveling up easy.
My idea is :
PLAYER MADE GYMS
When you beat the real GYM leader he gets replaced by a player.Now these would work like a championship,only 1 person can be the leader,you beat him BOOM you are the gym leader.That would be a nice challenge