Wow, you put 0 thought into this have you....

First, 15 different teams for the arena would be annoying for EVERYONE.

Second, what exactly constitutes a team?

The exact 4 ninjas? Welt, for a 9 tail naruto+healer build, let's rotate through the healers. And maybe let's trade roshi for a different char and rotate through the healers again. That should do it, the 6 tail naruto will probably carry the team anyway, a great help that is.

Every ninja on a winning team gets disqualified? Welt, newer player can't even complete the event then, they have to journey into ranked mode which makes it worse for them.

The first couple hour of the event tend to be bad, there will be people handing out free wins soon enough. Don't come complaining because you choose to play at a bad time and gets mad due to losing. Most ptw aren't evil enough to stomp around training arena for no reason, they gain nothing from doing it and have better things to do with their time.