If all 1st generation members graduate, I believe everyone will grow rapidly.
That's why I rather curious if all 1st generation graduate. In fact, I wish for it.
Maipuru was not much a reliable person until Saya-nee graduated. She has grown so much after she was appointed to be the leader.
See also Kaede changed when she is in ChocoLe.
========== First, let me state that I'm talking more about the tv show, not just skills on stage.
I believe Amimi is the best candidate for next leader. She could change Idoling!!! to the next level. Maipuru is rather an iconic leader that the group can rely on, while Amimi is full of idea, breakthrough, and most importantly, she has a lot of connections now that the group could use. Also she knows everyone's potential. Plus LesPros could become a powerful support for the group and is not over-protective to Amimi, not even to Ai-chan that even though she has become JJ model she can still go crazy.
Let's say if Rurika will stay for a few more years, she is good. Ogipro is not that over-protective to her, she is ok with dirty talk lately and she is funny. But I think she will end up to be just iconic leader like Maipuru. The group will never grown. Stagnant.
Let's say if Ton-chan will stay for a few more years, she is also a good candidate but her agency will surely over-protective. Plus she will go blind on dirty-talk topic, where Maipuru is famous of, where (I believe) the group is famous of (thanks to Masuno). But she has idea and breakthroughs.
Umeko and Sakacchi are lacking charisma, leadership aura. Hii-chan might be the real leader behind the scene right now, the only member who can yell to her juniors, the only member who can drive her juniors to run faster, but she is clumsy and moody.
Ai-chan probably just too young, but she is definitely the best 4th leader candidate. Asahi and Yurippe are definitely not on my list right now, probably not in lifetime.
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Talking about skills, Kadosawa P said "there will be no replacement for anyone". Clearly he is not going to get best singer out from audition to replace Maipuru or Ton-chan or Rurika. I enjoyed their chaotic Yarakai Heart, Love Magic Fever, Shokugyo idol anyway. He just got lucky out of 50 applicants, not to mention he had Shioya D back then. (I can still enjoy Tokyo Joshiryu for skills)
Idoling!!! is the group that frequently performs with 5-10 members, shuffling them. And without best singer, future songs will be balance to everyone. Everyone can perform any song. I rather choose that compared to now that some songs can be oblivious without Maipuru Tonchan Rurika.