Alright... finally my turn to speculate. Now that I can finally register... feel the wrath that is my pent-up raging speculation!!!
System
I'm almost positive this is going to go 360. As far as I know, though, Idolm@ster doesn't have any competition (Dream Club, I don't think, made a dent in any of their sales), and I think there's really nothing that's chaining them to the one system which any new fans probably don't own. Before I'm sure it was just development time that made them not develop for the PS3 (it was launch for these systems), but this time I think it's probable, slightly more probable if we're talking a year past the 360 release.
I find it interesting that an Arcade release hasn't been announced. I would personally do that at latest 6 months in advance, but if we're looking at an early 2011 release of the 360 version (and the fact that this is Namco), I would say that there will be no arcade release. With the recent trend to incorporate more character development and such (I'd even go so far as to say that DS was their experiment to see how much plot and casual they could bring in), I'm even more confident, since those don't go together well in an arcade.
Setting
I think 876 is probably in, in some form or another. As random idols you fight (name only appearances), I think it's as good as settled. As NPCs, I think it's likely. As actual producable idols... only a merger (765 and 876, not Namco and... uhm... you pronounce those the same, right?) can make this happen, I think. The question is, were any new people brought in by DS, and if so, is Namco willing to keep them? The answer to the latter is definitely yes, but I don't know about the answer to the former.
Given the pictures in the countdown, though... I think all our familiar 765 idols are in. Fans of any game will want to see more of their familiar faces doing what they do best in a sequel and you're pretty much forced to cater to an already existing, rabid fanbase. Of course, you'll have to add in new faces to keep things fresh. 10 idols already... given how I think the idol select screen should look like with more characters, I think two or three new idols would be good (random select can always take up a slot). One or two if 876 idols are producable.
If we were to break Idolm@ster free of its one-on-one arcade-style roots, though... it'd take a lot of effort, but more interaction between idols would really change the direction the series is taking to bring it more inline with the plot-based trends we've been seeing lately.
961 I would say is back. Kuroi's an already established antagonist that everyone loves to hate, and he hasn't really been developed aside from the fact that he knows Takagi, knows Kotori (or so I've been told), and that he's a massive jerk. There's so much more that can be done here, and I think this was on purpose. Kotori herself... is possible, but I think part of her appeal is that she tragically never gets to be an idol, despite... everything else. ;_;
So, my final prediction in this is: previous 765 idols are in, former 961 idols are in 765. Expect to see a lot more interaction between (very possibly) themselves and (definitely) the producer. 876 idols play a supporting role. Two new idols. No Kotori. (I have to admit this. ;_;)
Gameplay
I'd say the same as SP, with the possible exception that Idol Ultimate prelims would give more fans, and the game might force you to Idol Ultimate prelims if it would rank you up (so it won't burn a week). I could say more, but... Idolm@ster is inherently balanced, and anything just different from the norm would be put in Special auditions, I think. So expect to see new ones.
Performance lesson might be reimagined, since I think Namco would've gotten a lot of complaints on that one.
That's all my speculation before an actual (possible) announcement next weekend. My frothing demand for this game cannot be contained!
