As best as I can understand, this is a direct sequel to “BanG Dream! MyGO!!!” (that is a lot of exclamation points). The source post on twitter also has a short video accompanying it. They also announced two recap movies for MyGO that are coming out later this year. Here is the synopsis for MyGO from AniList:

The best way to fit in at a new school? Start a band! Anon has just transferred to Haneoka Girls’ Academy where girl bands are all the rage. But she started school late, so there aren’t many girls left to recruit. Then she meets her first friend and Haneoka’s misfit, Tomori, who’s hesitant to join. Still, Anon is determined to persuade Tomori and experience the true power of friendship and music.