Part of the strategy in creating an arena team comes from having to compromise. Having a sixth slot would drastically increase the likelihood of timing out of a battle and further reduce arena diversity without adding anything substantial to the gameplay.
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Definitely don't think they should alter current arena, but I see no reason they can't add another mode inside squad arena that does this.
If the 6th toon didn't come with a second leader ability, it would not be worth it to me because that is what really opens up new combos right there.