Yet another league idea!
Okay, so a few league ideas have popped up recently, so I thought I'd throw my own into the mix.
Each player is provided a pocket hopper which must be mounted directly to the marker with an elbow or vertical feed, and one bag of paint for the games that day. Players can use their paint however they choose, but if they run out, they're done, and the paint they have left at the end of a game gets added to their score in some way. (1 point for every 10 balls, or something)
To me this solves a few problems. The A5, mag fed markers, warp feed etc. are eliminated because the hopper can't be mounted directly. This might make those players mad, but it also prevents them from having an advantage. Aside from those, any other gun is fair game. The pocket hopper limits ROF and the amount of shooting that can go on in any given game. The limited paint and emphasis on conserving paint also cuts down on the shooting.
Ideally, this would be played on a large field with neither team in direct view of the other.