If JK Rowling ever decides to return to Harry Potter what should she write about?
Some ideas could be:
Years between the end of the last chapter and the epilogue?
Harry's kids going to Hogwarts?
The younger years of some of the popular characters
(James/Lilly/Sirius/Snape, Dumbledore, Hagrid)?
A story based in another wizarding school (Beauxbatons, Durmstrang)
These are just a few.
There are plenty more.