I think it could be cool to have gyms where you fight two leaders at once, with a friend to help you. (2 vs 2) It would be different and give more variety to the outcome of battles. I know that it is hard to get two leaders on for the same gym at once, but we could increase the number of leaders for the tag gyms to four to increase the opportunity.
p.s I may not have done this right, but it is my first post.