Yeah I thought about this too. Perhaps with extra stuff to do / fool around with, perhaps shoot paint at interesting stuff (like trees or things not normally in arenas) in the colours you like. Could be like a ball pit. Or maybe a training area that multiple people can join?
But maybe not immediately on release. Because of the Switch hype, there will be a lot of people will be newbies, completely unable to understand what such an area is supposed to do. They've never experienced that magical moment in the middle of a match, where you look an enemy squid in the eye, and you spontaneously start partying...
....like the other day I was in a casual serious turfwar match with a Splash 'O or an N-Zap, and getting 10+ kills each time, smokin' hot like a volcano. Always on the other team was a long ranger named 'Sam' I think, and he or she always ended up with 15+ kills. Boy, I thought, if we should be on the SAME team, we could get A LOT of kills in together. However, after many rounds, it never seemed to happen that we would end up on the same team! Frustration settled in. At one moment I was rushing at him/her for a kill like a foolish kamikaze (as one does) in Walleye Warehouse, and suddenly I thought "waitaminute why am I not being splatted?" He just stood there gazing at me. Maybe this person wanted to be on a team together as badly as I did? So then we started jumping around each other like fools.