[GMT]

If they're doing it to milk the cash cow, then it's going to be a recap movie. There might be a few new concert scenes in there to enhance the merchandising opportunities (though new concert footage isn't strictly necessary. They could just release new songs on the movie soundtrack.)

A recap movie with lots of moe gags would certainly bring in the otaku and encourage them to spend money on more overpriced merchandise, as it'd be a safe love-letter to the fans. There wouldn't be the potential of offending anyone like a movie with at least 50% original content would (<"Liek OMG, the girls aren't in their uniforms/my waifu has a boyfriend??? I'm going to burn down Kyoto Animation's HQ and then hang myself. Life is not worth living!"> -- future 2ch post translated from Japanese.)

The reader might go "But wait, didn't they do a good job with the Haruhi movie?" My answer is "The Haruhi movie was based off a small (albeit important) canon story, and KyoAni took so few risks that not only did they leave nothing out, but they padded the story with a bazillion hours of scenery porn. It's a three-hour-long love letter to the fans . . . it's a reason to have low expectations for a K-On! movie."

Who knows. They might prove me wrong. They might be willing to take a risk and fully explore a K-On-iverse story outside of the comfortable confines of the Sakura High School Light Music Club. Goodness knows there's a lot of post-high-school stories that could be told about the girls that would have some dramatic bite and general-audience appeal, but would also allow for cute slice-of-life/friendship/bonding/Mugi-vsion moments and lots of trademark KyoAni scenery porn.