I'm not sure if it's been suggested, but I think it might also be cool that if your relationship is high enough with an idol, and their rep or skill is high enough, you could instead manage their solo career since you've had such a great rapport with them. Perhaps they can even recommend other talent to work with your group if they enjoyed working with you enough.
I feel like we won't get totally separate groups before the release of the game, which is a shame, because I think that running an agency rather than a group makes more sense for the game's concept, and managing some solo idols would fit in really well with that. So I don't think we'll get it, but it'd be great.