The purpose of arena is to allow players to test new ninjas and lineups. I believe this is now more so for arena training than ranked arena, but this can be supported by that pop-up that always appear whenever you acquire a new ninja.
However, if we would like to try certain lineups with other mains, we would have to switch our mains for at least the next 24h (unless you pay for it), in which we might not even have a good lineup for. This pretty much discourages people from trying out non-meta lineups and to discover something out there for themselves. I believe there is a reason for such a diversity of ninjas, and yet half of them are barely utilised.
So, to put it simply, similar to the lineup simulator - or some sort, build in a function to allow players to switch between the various main characters to test out different lineups.
I don't know if anything like this is already present in other servers, or if it is even remotely possible, but if this is new, then please try to make it happen. Leave your thoughts below ^^