Travel through time to revisit old moments and characters. We've seen most of them in the films and the two leads are interesting enough to sustain a new adventure.
On the other hand Rowling has had time to develop those old characters through 7 books. The fans know who they are as soon as they appear on stage. The film Fantastic Beasts and Where to Find Them was at a disadvantage when trying to cram a novels worth of new characters into a 2 hour film. Half of them didn't get the chance to make a real impression. She might have been better off there focusing on her two leading couples rather than introducing about 7 or 8 antagonists across multiple subplots.