I've been thinking over the issue with new mission types, and i cannot help but think that this would be a very efficient tool in your quest for balancing ATs. Instead of (or in conjunction with) changing the powersets, wouldn't it be great if the "balance" depended on mission type instead of powersets.
For example, a scrapper might have a natural advantage when the goal of the mission is to beat up the bad guys, but what if there was a mission where stealth or detention is of the utmost importance and you get bonus exp for completion of the mission if you do not "arrest/kill/whatever" the bad guys?
I think missions such as that would add some nice flavor