I think Cinderella Girls is too focused on being its own entity to have any part in mainseries iM@S. The 765 girls were there at the beginning to reel people in, but it was the new characters that got people to stay loyal to it. Million Live obviously plays a heavier focus on the main girls but I'm not sure if I see them making it into the games either.
CG seems to be fine where it is, seeing as they're starting to celebrate their 3,500,000 registered accounts celebration (how many of those are still active and weren't illegitimate farm accounts is unknown), while I don't think Million Live has even reached 50,000 (46179 at the time of writing, judging by highest valid user ID number as these numbers started from 1 onwards) registered accounts yet. Not sure what Namco is going to do to boost ML support, but it's obvious that ML isn't anywhere near the pace that CG had when it came out (I wonder if their gaijin deterrent AKA IP locking played a role, though that's easy as hell to bypass and we have a ton of gaijins playing KanColle).
I personally don't want to see ML in mainseries iM@S, I think it's fine staying in that spot where aniM@S related stuff go as long as Namco figures out how to get people to throw money at the damn thing (maybe the character designs weren't unique enough? I can only name Shizuka and Mirai off the top of my head just because they seem to be the equivalent of CG's Rin and Uzuki). Same goes with CG, it needs to focus on being its own entity with its own following. The amount of merch and manga it's able to push is incredible.
One point a friend brought up however, was that the mainseries seiyuu aren't exactly getting younger year after year. A couple of them have children and age will eventually take a toll on their voices, right? What's going to happen in the inevitable future when they may have to retire? We're sure as hell not going to have everyone pull a Yukiho. Either iM@S will have to come to a bittersweet end, or we may end up having to say hello to some new mainseries characters.