This is a bit of a weird question...basically, I mean, where can it go next?
It started out in sincerity: girls in cute outfits using their powers to help people. Then, of course, with PMMM and all its successors, it began to take an "oops! all darkness" turn where you'd lure people in with girls in cute outfits, only to hand them a story of existential horror and despair.
But we've already gotten to the point where that's overdone...so what's next? I mean, the 'sincere' version of the magical girl is still alive and well; where should authors go now if they want to subvert the genre and deliver something new and fresh?
The industry answer seems to be comedy...I never watched Machikado Mazoku, but I think it tried to do something like that. And there are some other magical-girl-themed comedies I've seen that are just straight-up insane Japanese humor at its finest~.
The problem with that is that most magical girl anime is already supposed to be funny, at least some of the time...as the target audience is preteen children, that's pretty much a given. So it's less of a subversion and more like doubling down on an aspect of the genre that already exists...
So here are some of my ideas (AKA what I would do if I had time for a new magical girl story rn)
-Just change the atmosphere
Like, what if 'dark magical girl' was actually 'DARK magical girl'? Not innocent moe babies being sacrificed for the greater good, but older, more mature young women who are just trying to survive a tough supernatural job.
Or even without aging up the MCs, imagine a teen magical girl story with the ambiance of a Batman comic (or, dare I say it, a Deadpool comic). Is this kind of stuff being done? Because I've never seen it, and I think it would be really cool.
-Or change the characters
This is the kind of thing I'm already doing with the magical girl concepts I have...it's amazing how you can breathe new life into an old trope simply by changing the type of person it's applied to.
Instead of the 'average girl with a happy home and a couple of school friends', what about an MC who isn't an average girl? Or one who doesn't have a happy home? Or any school friends?
Not in a dark, angsty, 'CW: abuse/suicide ideation' way, but just a 'this is just how my life is, and I kinda have to deal with it' way.
The typical 'dark magical girl' leans into the problems of a girl's life, making them extreme and/or allowing them to warp the character into something twisted.
But plenty of people live fairly ordinary lives despite having issues...maybe they've carved out ways to handle them, maybe they ignore them and focus on other things, maybe they just aren't affected by them the way other people would be.
Basically, there's middle ground between having a scrapbook-worthy life and living in a waking nightmare, and very few magical girls are found there. I'd say the vast majority are 'reformed' characters that start out as antagonists, and are rarely focused on as MC's. Maybe it's about time they got some spotlight.