I'll start by saying that I love Grand Arena as a feature and hope you guys keep running it on a frequent basis. Two a month would probably be ideal, but I'm sure that's going to be tough to balance a...
Easier than all this. Have seasons and use a ladder system within GP groupings. So if you win you get say 20 points, 2-5 get 10 points, 6-9 gets 5 points, 10 gets 0. These points along with GP determine your position and who you'll be up against in your next set of matches.