My guess is, they wanted a nice, round number for the "lead" of this group; Rin was 01, and now Uzuki will be 10.
Now, whether Mio will be 11 or 15 is still up for grabs.
Also, I'm 80% certain that we'll only have one cute next time around. My guess is, it'll be Miho. I'm also guessing that our Cools will be Nao and Riina, since they do seem to be exceedingly popular. (Though I kinda want to see Reiko Takahashi, just out of curiousity. Also, she's old enough that, in my theoretical, never-been-shared prequel, she'd be roughly 16-ish, prime idol age. I just don't have a song for her like Mai and Kotori have. This would fix that without resorting to unused Radio songs.) Add in Mio, and that leaves one other passion. I REALLY want it to be Yui, but that seems unlikely. I think Airi or Natalia have better chances than her, Natalia more likely since she seems popular.
So, guess for next wave (if it happens), in no particular order:
Mio Honda
Nao Kamiya
Riina Tada
Yui Ohtsuki
Airi Totoki or Natalia (Whatever her unrevealed last name is. I mean, we've had other foreign last names. I'm looking at you, Mary Cochran and Cathy Graham.)
Miho Kohinata