I voted for Mizuki and Haruna.
Haruna, because as long as I mention her here she won't graduate. Mizuki, because it's time. Give Ikuta a year or two as leader, please.
I also thought of Masaki, same points as Mukuro, but I don't know. Wouldn't be surprised, honestly.
Reasons for every member to graduate:
Mizuki: UF would have to drag her out, but she doesn't really bring anything to the group, voice aside, but she's dropping. Will work for m-line and does occasional events and photobooks.
Ikuta: Becomes a TV regular or something. Alright, she's gonna stay for some time...
Haruna: She herself thinks she's getting too old. Once she's 25 she's out.
Ayumi: Will become the next Shimizu Saki with doing choreographies and stage plays.
Masaki: Her back. Will officially leave showbiz and write lyrics under a pseudonym.
Sakura: Will try solo stuff. They give her two years and realize the market is dead.
Oharu: Graduates when she's done with university.
Granted, everyone from here on is unlikely to graduate soon imo.
Miki: I have no idea why she should graduate, honestly. It's all great for her at the moment. Becomes UF president or something.
Maria: Scandal, off to marry some baseball dude or something.
Akane: Is found out to write dirty AyuDuu fanfic and therefore kicked.
Kaede: Will get frustrated that after all that work in the KSS she's still only a background member in MM. Considering her age she'll probably graduate in her early 20ies which isn't that far off.
Yokoyan: They realize she's just Kamei Eri who sneaked back into the group.
Chii: A combination of post-CG sadness (especially once they dissolve them in one or two years) and the stress Up-Front puts on every MM "ace" will make her leave early.