I don't know how'd this be a Royal game as there are turrets and presumably a nexus, which causes us to, ya know, respawn. Though this would be a pretty cool project to work and play on. A 20man Summoners Rift. Everything would be chaotic.
a) what is the winning condition? b) how to prevent teams from teaming up and fighting a 3v1?
This would not only be chaotic as hell but it'd be extremely hard to balance!
I'd prefer a 4v4v4 map. Blue, Red/Purple, and Green (or Yellow?). Hexakill exists, so a 12 man map is okay for the game's standards. A four team game might be taxing for people playing on their laptops.