Time to do some speculating again.
I'd say the group is divided into two groups: Those that could announce their graduation tomorrow and nobody would be really surprised and those that most people don't see graduating anytime soon. The line goes right between 11th and 12th generation, with a few exceptions I guess.
1. As others have mentioned, if someone graduates this year it's probably not going to be Mizuki. Management would want to give her a full year of H!P leadership - if nothing extraordinary happens, but that's unlikely.
2. Sakura is the group's best singer and really nobody comes close. They wouldn't let her go before they add another good singer - and started pushing that one right away. I'd see it as similar to the situation when Sakura herself joined and Reina was on her way out:
Sakura joined in September 2012, Reina announced her graduation in November of the same year and graduated in May 2013.
The new members will probably join in June this year, which makes me believe Sakura would graduate at the end of the spring tour 2020 at the earliest. It's possible that she'd leave at the end of the fall tour too, or leave without another good singer being added to the group, but I doubt it.
3. Chisaki and Kaede are always weird cases in my book due to their age when they joined the group. They're both already 19. Granted, that also means they decided on a career in idol business instead of going to university so they definitely have time until they're 25 (unwritten, unofficial rule). But seeing that Kaede is always in the back I wouldn't be surprised if she called it quits before that.
Which leaves me at the following conclusion:
We will have a graduation at the fall tour, and the most likely to graduate would be Ikuta and Ayumi - maybe together, even. They've both more or less reached their peak and can't go much higher in the group. They've also been in H!P for 8, Ikuta even 8 1/2 years.
It's also totally possible that Masaki will graduate but she's so unpredictable that I don't know where to put her.
Management might decide to book Yokohama Arena for a double graduation, too. This would be practical since Budokan might be closed for renovations by then.
My timetable would look like this:
- Spring 2019: Nobody graduates, I think they would have announced something by now if so.
- Fall 2019: Double graduation Suberi combi at YokoAri
- Spring 2020: Oda Sakura graduation
- Fall 2020: Fukumura Mizuki graduation; Mizuki will have hit her 10th year with Morning Musume by then.
- WILD CARD: I don't think Masaki wants to be leader. That's why I think she might graduate either before Mizuki or right after her, 1) pulling a Riho and graduate at a small venue at the Countdown concerts, 2) graduate at the H!P winter concert, 3) graduate sping 2021.
- Spring 2021: If it's not Masaki I think nobody will graduate here. At this point the senpai are gone (as said, I think Masaki won't stay long after the last member of 9th/10th/11th gen) and management would want to give the "new generation" their spotlight. Think Sukatto My Heart promotion.
- Fall 2021: If nobody graduated during the spring concert - I'm pretty sure they want to keep up with the "1 grad per year" cyclus - this spot will be open. This is in 2 and a half years, many things could have changed by then. Maybe even one of the girls who are supposed to join this year decides to graduate then.