Bill Cipher could potentially be involved as well. Since he is often associated with chaos and mystery in Gravity Falls, the drifting stars could be part of his grand plan or something he is interested in manipulating. His presence would add an extra layer of danger and intrigue to the fanfiction, as Dipper, Mabel, and the others would have to deal with his devious schemes while also trying to understand the drifting stars.
Stanley and Stanford Pines might also be important characters. Given their knowledge of the town's secrets and their past experiences, they could play a crucial role in the 'drifting stars' fanfiction. They might have some prior knowledge about the stars or be able to provide some guidance to Dipper and Mabel as they explore the mystery of the drifting stars.