So, first off I want to note that I recognize creating a brand new AT is... tough. Like, even with all the animations and powers taken from other existing sets, it's a big undertaking. So I know this isn't something that's even remotely likely to happen. (I would love it to, which is why I'm making the suggestion, but, I recognize the difficulty, so I don't expect it!)
That said...
How cool would it be to have a variant on Mastermind with Scrapper and Sentinel* primaries or Dominator secondaries, as primaries, and MM primaries as secondaries, and with the MM attack powers from the primaries replaced by some modest Defender/Corruptor set powers?
This is just a super rough example, but I think it gives a good idea what I'm thinking here
Katana/Mercs
Katana - As Scrapper, but with Calling the Wolf replaced with Placate.
Mercs - Replace Burst, Slug and M30 with Triage Beacon, Shield Generator and Acid Mortar
Inherent - Avenge the Fallen -
Whenever a MM pet under control of the player, or another player on the team, dies - gain health, endurance, defense, accuracy and damage, proportion based on the level of the deceased.
Ex: Minion - Half a small Insp. Lt. - Small Insp, Boss - Medium Insp - Player - Large Insp
The idea here being two-fold -
1) You generally lead from the front, and while you have some ability to defend yourself and your team... your best defense is going to be making the enemy dead first.
2) Because of your limited defensive options, your pets are going to die and need to be re summoned fairly regularly - Avenge the Fallen compensates for this by making inevitable losses advantageous.
Basically, you're individually weaker than most heroes or villains, but you have back up and your team work lets you overcome obstacles. A Mastermind will have stronger pets and support options, but you've actually got some teeth on your own if it comes down to it.
The reason for suggestions such a diverse array of offensive powersets is simply thematic - Assault sets provide a nice variety of options for range of engagement, the pure ranged sets let you play a little more cautiously, and the melee sets, more aggressively - with damage tuned to the set in question.
Ultimately the goal is to be an aggressive, engaged team leader, as opposed to the more detached and defensive play style of the Mastermind.
It's just an idea.
*I specificy Sentinel rather than Blaster because I think the slight modifications made to the Sentinel version of the sets would generally play better with this kind of set up. That said with this being a new AT it may be that there are powers for every set that would need to be swapped out. Maybe all these sets could become Assault-esque sets, with maybe Katana coming with throwing star powers, Broadsword with Bow powers, AR and Beam Rifle having some of the gun-butt/bayonet attacks from Arachnos Soldiers, stuff like that.