If I was advising the group, I'd actually have her focus more on developing her personality over singing and dancing.
Honestly, while her singing can be painful she is still a better singer than Sayumi, Koharu or Konno. And she's totally willing to adapt a character that's not cool, a character that can be made fun of, something that could totally work for an idol. And I agree with you, her personality is probably her biggest potential. She could easily go the Koharu route and become an anime character. That's what I mean with management going the easy way: Of course it's easier to have Maria as one of the top handshake sellers and sell a dozen photobooks of her than to actually promote her in an individual way. Hell, if Sayumi would've done nothing but performing she'd probably be another Oharu now. I get that they can't just say "hey, who needs a shoujo anime heroine - we got a girl to promote!", but they could do something to build her character. May it be the one of a spoiled princess, or one of the youngest daughter who is a crybaby, or whatever. Make her do a sketch about that on Hello! Station or one of their damned DVD magazines (instead of the 100th episode of musical chairs). You don't need much. Let her do mock interviews where she misunderstands everything and makes herself the focus of attention.
I thought Akane's fanfiction during Duu's graduation (was it that concert? I think so), about Duu dating Miki but dumping her for Ayumi, was funny and interesting and definitely is something that I think of when thinking of Akane. You can't do this only once per year, though. And not with only one member.
Kaga is probably all she's ever gonna be, but she's acceptable filler and is so damned likable.
I like Kaga, but your description underlines my opinion that until a few years ago, Kaga would have never been chosen for Morning Musume. She has peaked already, and while she has some room to improve it's not much. We all shook our heads when girls like Tsuji, Konno, Sayumi, Koharu and the like were added to the group but in the end we could follow them developing into sucessful idols. I fear that the most spotlight Kaga will ever get was a few days of headlines about her cinderella story when they introduced her as a new member.
Then there's Chisaki, who is 18 and therefore unlikely to improve on her limited abilities, and seems to hate being in MM. In short, she's everything that Yoko is not. She could be fired tonight, and I'd be totally cool with it.
Gotta be the dick here: Does Chisaki have any personality aside from being shy and getting red ears? I don't think she adds anything to the group except for a pretty face. Her dancing is pretty good I guess but that doesn't matter since group dancing can always only be as good as your weakest link. They call her the Sayashi of Tochigi, but she doesn't even have half of Riho's stage appeal.
I'd love to see the current leadership completely gutted, Maa made the new leader with Ayumi as her right-hand, and one new honest talent brought on board.
Knowing Masaki I doubt she'd want to be a leader. In fact she's hinted that she originally wanted to graduate this year but didn't when Oharu announced her graduation. She also mentioned that she'd like to be the group's producer - I know they won't give her much responsibility but if she leaves now, goes to university for 2-3 years and comes back as a advisor I'd totally welcome that (ignoring that she's probably already decided not to go to university).
We scoff at the idea of MM becoming a talented group ever again because the J-idol world has moved away from that so much, but all you have to do is look across the hall at ANGERME to see that it can be done, and how good they can be. Sure, ANGERME sells half of what MM sells, but that's the difference in branding. If the current lineup of ANGERME was called Morning Musume, they'd sell better than they do.
Which amuses me even more considering the bickering between Morning Musume and ANGERME fans. MM fans are elitists after all, ignoring the fact that ANGERME has no real weak link.
/Edit: Because we were talking about it: Oharu's graduation concert.