The real problem with Mercenaries is that it was probably the FIRST Mastermind powerset that was built.
That means that what Cryptic Studios had "available" for being repurposed into the Mercenaries themselves was OLD LEGACY powers that had been used by Assault Rifle for Blasters and were basically the Tommygun attack powers used by a lot of Family and Mooks NPCs. Later on, Assault Rifle for Blasters (and Defenders, eventually) got a "tune up" to be less stupid with their animation times and recharge and so on ... but that update to the tuning of powers never extended to Mercenaries, which had to (polite cough) ... soldier on ... (ahem!) with their poor game design after everything else that came after them were designed much better. So the Mercs' attack powers are all OLD LEGACY powers that were meant for NPCs, where no one is going to mind if they underperform which have never been given a balancing pass to bring them up to par with the rest of the Mastermind primaries.
In other words, the real problem with Mercenaries can basically laid at the feet of being first NEVER getting adjusted to match everyone else ... leaving them stuck with obsolete antiquated terrible attack powers that work "fine" for Foe NPCs (who tend to be more numerous so as to "make it up in volume" for their lack of individual firepower) but which land on absolutely abysmal breakpoints (that all break unfavorably away from Mercenaries being useful/effective in combat) ... and they have never even once received a balance pass to bring them up to snuff.
Cryptic Studios never did it.
Paragon Studios never did it.
... and now it's up to Homecoming to finally get around to doing it when they can find the ever scarce resource of Developer Time™ to do it.
Personally speaking, I'm of the opinion that twiddling some of the recharge times on the OLD LEGACY obsolete powers along with basically DOUBLING the base damage of every damaging power that Mercenaries have(!) ought to be sufficient to bring them up to par with the other Mastermind primary powersets. That way, you would only be editing the excel spreadsheet parameters that govern what they're "allowed to do" rather than trying to reprogram all of their animations and deal with the huge can of worm that THAT would open for things to start breaking all over the place. Yes, it would mean that the OLD LEGACY obsolete animations would be kept, but at least they wouldn't all be AS SUBSTANDARD as they currently are.
Point being, if you can "better balance" the animation to recharge times for the Mercenaries attack powers, so as to reduce idle time waiting for attacks to finish recharging, while simultaneously increasing the damage that each power delivers ... that then would cause a marked enough improvement in the damage output of Mercenaries to keep them from gnawing a hole through the bottom of the barrel like they've been doing ever since the release of City of Villains.
After that, you basically just need to fix Serum into not being a Joke/Trap power that's a complete and utter waste of a power pick ... and you'll be good to go.
I'm thinking that would be the simplest, cheapest and perhaps most effective way to "fix" Mercenaries so they aren't weighed down as badly by the legacy of their abysmally bad game design parameters that they got stuck with in the first place which have NEVER BEEN MOVED.
Is it a "perfect" solution? No.
Would it be an "adequate" solution? I'd like to think it would be ...