^To be honest with you I don't think it's a very wise idea to make someone from the 13ki (someone new) the new ace after Riho.
Not to be rude, but if you've been a member since the first generation, then surely the concept of someone new as "the new ace" shouldn't be foreign to you.One name: Goto Maki.
You make a good point there, and it's my fault for not explaining what I meant properly, which I apologise humbly for. Yes, I am aware that Goto Maki was made the ace as soon as she joined Morning Musume (even though Abe was still in the group and also the ace at the time), but at that time Morning Musume had only been around for 2 years and only just debuted as a group the year before. Before Gocchin joined hardly anyone paid any attention to them, but after she joined and they released Love Machine and gradually worked their way to a Number 1 single they eventually established themselves as a well known Japanese idol group.
Fast forward to 2015 and Morning Musume has been around for over 18 years. They have enough fans that know how the system works. If they make someone completely new the ace it will not go down well. She will either get compared to the other members who've been in the group much longer than she has, or she'll be expected to be the star of the show from the get go and it will be a repeat of both Goto Maki and Sayashi Riho all over again. We can't say why Gocchin left or what Riho's real reason for wanting to graduate (if she has one), but maybe putting all that expectation to be the star of the show and always expected to be perfect on someone new isn't such a good idea because it results in the girl leaving early after 3 or 4 years. I don't think Up-Front will want to repeat that anytime soon.
That being said though, they probably will eventually do something like this again with whatever generation they add in 2017 or 2018:
Goto Maki - Joined Momusu in 1999. Made the star of the show from day 1. Left in 2002
Kosumi Koharu - Joined Momusu in 2005, six years after Goto Maki joined. Made the star of the show from day 1, although kind of failed a little. Left in 2009
Sayashi Riho - Joined Momusu in 2011, six years after Kosumi Koharu joined. Made the star of the show from day 1. Leaving at the end of next month.
If 13ki get added to the group next year I can see someone from 14ki when they're added in either 2017 or 2018 being made the ace or at least put in the spotlight from the minute they join. This may not make a lot of sense, and I apologise again if it doesn't, but by that time someone who is already in the group will be doing ace duties and can help mentor them in their role as an ace just like Ai-chan and Reina did for Riho
At least to me it seemed that way even when Gocchin and Koharu joined. Thinking about it now I probably just don't want someone new to be made the ace or put in a position similar to an ace role in the group without there already being someone in the group in the role of the ace who can help mentor them and help them settle into their role in the group *hopes this makes sense*
Edit: Sorry about the long post and response. I can't seem to explain myself or what I'm trying to say very well in less words that often