There is currently a trilogy in the works based on 'Magical Beasts and Where to Find Them', which is a book of about 40 pages. I think though, if they wanted to, they could just go ahead and reboot the original. Sure, it has only been three years since the second 'Deathly Hallows' movie but hear me out.
Originally, J.K. Rowling wanted Terry Gillian to make the movies. The studio, fearing that the films would be odd, not terribly kid friendly and expensive, vetoed the idea. But, the original Harry Potter fan base are all adults now and I think they would pay for and enjoy a Time Bandits/ Adventures of Baron Munchausen / Imaginarium of Doctor Parnassus style Harry Potter.
I also think it works as a anime or cartoon - instead of, or in addition to, trying to turn 40 pages into three movies maybe they could make a kids version (animated), a young adult/teen version (the original) and a grown up (Gilliam) version. I'm pretty sure that most fans would watch all of the above.
This one, from Nacho Punch is meant to be funny, but it gives you an idea of how well it would work.
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