I REALLY dislike the idea of Imas 3DS, simply because it's a region locked console. That, and I can't imagine 3D would be too useful to the series as a whole. That, and SP sold better than DS, so I'm guessing Namco will go for the safer system.
Also, if 876pro gets more idols, its likely to get Cineria/Ayane and Yumeko, at least as "unlockable" idols. Both might require another idols story line or some such... Other than that, I don't see them adding too many others. All of the portable imas games, though less restricted than a new one would be, have only featured a small amount of idols: three in the SP games + one unlockable and three in the DS game. I think it might become a pattern to off set the character-happy console game.
Though Shinkan Shoujou might show up, if their appearance in the 360 coding is anything to go by. Also, a reference to Rin, Kirari, and other CG idols might happen, as it would be the first game to be published post-CG. Just a thought.