Add a taunt to all Mastermind primary power pool attacks that starts at a base of 0 magnitude, but then scales up based on the number of minions who are within superiority range and set to defensive/follow.
This can use the same code path to calculate validity as the existing health sharing check that already exists for masterminds.
Reasoning
When MMs were first introduced, they were touted as the villain tank-alternative. However, Brutes very quickly took that spot for a number of reasons - ease of use in the role, defensive secondary, and the fact that their core mechanic is very much about being an attention diva. For people coming across from CoH, the similarity between the Brute and the Tank made the comparison easy.
Masterminds have since been left with a bit of a questionable team role. There's no question that a MM can solo like a champ, but in teams they often end up as being a bit defender-ish with weaker support but more numerous (and sometimes irritating) attacks. They end up being a quasi-defender that, honestly, a defender or corruptor is probably better suited to perform. What they bring that a defender or corrupter doesn't is personal survivability.... sometimes. They also bring minions, which are controversial enough that some MMs will intentionally run without their class-defining feature when in large groups.
On top of this, Masterminds have superiority - complete with the health sharing mechanic - which is all about survivability. Survivability is a trait you want in someone filling a tank role, but a tank role is barely viable if you can't actually attract the attention of anything.
While it's true that a MM could grab provoke as a pool power (and it works!), it seems a bit weird that a pool power would be a necessity to be able to effectively exploit a core AT ability in a group situation. For this reason I suggest doing something about it, and by having the taunt scale based on superiority it means those MMs who want to tank things can, while those MMs who prefer not to aren't suddenly landed with a taunt they don't want.
Solo Impact
None. It's tied to an attack already, so if the MM was drawing agro, they were going to do it anyway.
Team Impact
MMs can grab agro! Because it's tied to how minions work, it means a MM can control how much agro they're grabbing. By using a smaller number of minions, they can set their taunts to be lower magnitude than the tanks or brutes, making them great for off-tanking or similar. By running with more minions, masterminds can experiment with more central tanking roles - although it's likely MMs will need more than just a taunt for tanking late-game content. Even so, being a viable off-tank is still a useful role to fulfil, and doesn't step all over the toes of ATs like the Tanker or (to a lesser extent) the Diva (Brute).
The taunt effectiveness is tied to the attack itself, so for many MMs their taunt will be single target rather than group based. This immediately changes how they interact with enemies, and sets them well apart from Tankers (who are all about the AoE taunt) and Brutes (who are all about the attention from everyone!) - making them a much more strategic, scalpel-like tank. One might even say... a mastermind.
As MM minions die, their ability to effectively tank wanes. This puts MMs in a position of being able to relief-tank (stepping in and taking over from a main tank for a short while to provide the MT with some brief respite).
Love ya, Brutes!