Harry Potter 9 would so gangbusters for sure but they need to wait another decade to get full nostalgia. Wait until 2031 which will be 30 years since the first movie. In the meantime, they can pitch ideas as to what the next story could be, but as for the people saying the epilogue closed things indefinitely; the Cursed Child is a bad thing and all but it took place, a year or two after the epilogue?
Anyways in the meantime, they could do a Harry Potter animated series akin to Star Wars the Clone Wars. Maybe even a full adaptation of the novels in cartoon form that include all the elements cut out from the films such as Peeves and expand on the original story. That way a new generation can see and grow up with the show, and the rabid fanbase (which for some reason want HBO Max or someone like Netflix to redo the novels in live action again but as a tv show) can shut up and enjoy the things they read from the books fully adapted to screen.
As for Fantastic Beasts, I feel like they should dump this on HBO Max and if it’s successful enough somehow, do the last two movies as a miniseries ending and call it a day.