Neither Fukumura or Oda have much opportunity for continued growth, as things stand. They're both stable singers/dancers/MCs, and I feel like either could decide to challenge a solo career. This has been a point where many of the long-tenured front girls decide to dip.
Ikuta has already talked openly about waiting until after the Olympics to consider it (and, well, those are over now). She also has enough experience in various fields (tv, magazines, radio) and connections to do anything she wants.
Ishida, though she has expressed a desire to not be leader, I think is actually quite suited to it. A "reluctant" leader, perhaps? She also seems to have a blast with the younger members. All that said, she is the 2nd oldest and is pushing 10 years in the group. She also has tons of TV and radio experience, and has enjoyed stage work in the past. I feel like she still has room to challenge herself, but that age/tenure combo are pushing the usual limits.
Sato has the obvious recurring health issues, but like I said above, I think management knows she's their best seller and (though they do little to promote her individually) will bend over backwards to keep her as long as possible. It comes down to whether she needs to stop for health or can physically continue. I think she'll continue as long as she can. I'd give her another couple years.
Nonaka, Makino, and Haga - I think they've patiently been waiting for their turn at bat, but are all capable of graduating to pursue other ventures. They've been in the group a respectable amount of time, and not everyone stays 10+ years. All 3 still definitely have growth potential, but if Ogata can graduate then so can any of these three. Haga in particular has really been having a glowup the last couple years.
Kaga and Yokoyama joined the game later in their teens, with Kaga already having had 4 years as a KSS under her belt. I think there's still a couple years at least for both of them, especially since idol age limits have been increasing.
Morito ... another late comer to MM, but with 9 years total under her belt so far. She has more H!P tenure than 12ki. I personally don't think her voice sounds that bad after her surgery, but health is always tricky with such a profession. She seems to have finally settled in, and I could see her going for another couple of years. After all, not everyone necessarily wants to be a lead singer and are happy to just be there and have a few solo parts. She's totally a wildcard here though given the health thing.
I'm not even going to consider any of the 15ki. They're too new. Okamura has recovered her hearing, and so long as they keep that in mind going forward, there's no reason she can't continue for a long while yet. She's the only one I'd keep an eye on, tbh.
Since with Hina Fes performance, I felt really sorry for Chii with the sound of her voice. I know she's recovering from polyp removal from her vocal cords, right? And her voice hasn't been too prominent in songs recently.
