Jo claims to have a lot of notes she made while writing Harry Potter. The best thing she could do with it is to publish her notes, doodles, and all that crap adding some of her thoughts and explaining how she created the whole Potterverse and how the whole adventure was. Honestly, it'd feel like a more honest book and a better approach to explain her fans the real origins of the story. I've seen her drawings and they're cute. I'd definitely buy that book instead of whatever she's trying to do now.
Her worldbuilding isn't that bad. What she offered was enough to explain the story we were given and that's it. She never intended to create more than her books, but then, I'm sure she never expected to have obsessive fans who couldn't move on from the books. The worldbuilding in Fantastic Beasts isn't that bad, but it's childish and repetitive if you're a person who's been exposed to more different fantasy worlds beyond Harry Potter. You can feel how she's out of ideas and she seems forced to create and repeat what she presented in the original books but she can't. I honestly don't blame her for that, but I don't feel sorry for her because she agreed on making these movies.
I know we often make fun of GRRM for taking so much time writing, but his World of Ice and Fire book is exactly what Pottermore should have been. It's a collection of stories from the point of view of a Maester who's trying to educate a young prince on history. We don't need every detail about Aegon I, only how he conquered Westeros and we have that. JKR should just take a whole year working on something similar and move on. In fact, Fantastic Beast should have been an anthology rather than a story about a man nobody really cares about.