Possible offline four-player multiplayer mode is suggested with the line "Even though it may be offline... I want to play multi-BOY!", and new music is also implied with "Guiter music are good but music from castanets, triangles, and maracas is something I'd like to listen."
Yes, the script on the page spells guitar like that.
Namco declined to comment on any updates when approached by Eurogamer, but the game has 47,685 players so far and GIRL has already reached the moon--hopefully that's enough to give Keita Takahashi ample reason to create more content.