The problem of unauthorized book sequels have been with us since the beginning of copyright. Charles McGrath brings us up to date on it link here
. By his account, if the originator doesn't object, the sequel writer can get away with it. If not, he can be made to pay or be banned completely. The problem seems to be showing that the sequel is transformative. That obviously is in the eye of the beholder and judges don't seem to be very sympathetic and may not recognize the point. It is clear that the sequel writer wants to capitalize on the popularity of the original but is also paying a form of homage and sends readers back to the original. Unfortunately, author ego gets heavily engaged and appears to prevail in the courts.
It would be hard to write a clarification of copyright that might serve the public interest as well as the originator's. To me it seems that the possible net harm of a sequel, even a bad one, is so small as to warrant allowing them all.
It would be a good move, short of drastically shortening the validity of copyright or abolishing it entirely.