I honestly don't see how this is a problem, in WoW there was a thing called *ing which is staying at a low lvl with OP Gear for the level bracket you were in which Blizzard didn't frown upon, so what makes it unfair exactly? I mean it isn't an exploit cause nothing in the rules say you have to level up you know what I'm saying?

Honestly, it should break down by power only regardless of levels. I say this because levels do matter to an extent regarding talents. But I'd break brackets like this.

5k-10k
10k - 20k
20k - 30k
30k - 40k
50k-60k
60k-70k
70k-80k
80k-90k
90k-100k+

Then the higher the battle power, if there isn't enough players on average in that bracket range, it'll reduce the amount of players needed to join but the minimum would be no less than 5 on each team. And even with a 10k difference in power you can still win. By either having better ninjas or better placement. :D