I'm really excited for the movie, but hopefully it won't end up being a stereotypical plot focused on Haruka, Miki, and Chihaya meltdowns that occur at different points in the story with hardly any screentime for the other idols except for comic relief like the anime basically. It would be cool if it was about them going on a concert tour and we get a lot of cool concert scenes...then again that's a bit stereotypical too, but they're cool. Hopefully we'll see some new songs too.
With those three being that prominent on the artwork, I'd say the chances of it focusing on any other idols than those three are slim to none. In a 90 minute movie though, you do need some focus - you can't give everyone equal time and prominence in the spotlight (else you just need to be a better scenario writer than anyone who's ever lived).
I'd be willing to bet we'll be seeing a new song or two, if only even an OP and a main theme. Heck, both an OP and a main theme could be one and the same if people really feel like skimping on the cash... and oh yeah. Namco's a part of this. Hmm...
Dengeki site says this won't be a retelling of what we've seen before... but it was announced at the Winter Festival concert, and I haven't seen a date drop for this yet. What do you suppose they'll announce at the anniversary concert?