I honestly have a feeling the next game will focus on the Million Live girls. It seems like they're going to make a big change and it seems like its about time for a cast change. They basically forced the Million Live girls into the Idolm@ster movie and the movie had a lot of focus on the Million Live girls needing to develop and striving to be like the 765 pro girls who are already at the peak of their careers. There are less Million Live girls than there are Cinderella Girls by a long shot. The Million Live girls have more development/personality in general. All the Million Live girls already have voice actors. Not to mention there has been quite a few Million Live girls being released as dlc for One For All. Definitely more than Cinderella Girls.
So as far as I can see, this next game will either be focused on the Million Live girls solely, mix the Million Live girls with the 765 pro girls (which the songs being released of the Million Live girls and 765 Pro girls singing together definitely brings this to mind and it would be easier to ease people off of the 765 pro girls this way), or just be another 765 pro game.
I would be surprised if it was focused on the Cinderella Girls and straight up shocked if it was focused on Male Idols (ew) the main Idolm@ster demographic has always been men and obviously men want to produce girls in most cases so I definitely don't see them going that route ever, or at least a long time from now. A mobile game makes sense because honestly there are a shit ton of girl "gamers" who play nothing but mobile games so ofc they will get a good amount of girls playing it; but a main series game? Naw.
With Cinderella girls, yeah the anime is coming out, but a lot them don't have voice actors and they're not really in depth characters at all and theres A LOT of them so I don't see that either. (inb4 its Cinderella Girls and its focused on the girls appearing in the anime and all the others are DLC lololol). Either way I'll only really be disappointed if it focuses on male idols. (and hell I'd still buy it)