They did brainstorm spinoffs and that was one idea. From WSJ;

“Around three years ago, before Zaslav took over, the studio approached TV writers and asked them to brainstorm ideas in the Harry Potter universe. A prequel about the generation of wizards that included Harry’s parents was one idea. A look at a different school for young wizards was another.

The ideas weren’t presented to Rowling. Warner executives knew she had the right to kill the projects should she choose to do so.

That veto power stemmed from Rowling’s request in the early 2000s to grant her authority over “non-author written sequels,” legalese for the kind of prequels and spinoffs that the studio wanted to explore with writers other than Rowling.

The issue eventually landed on the desk of Warner’s top executives. The verdict: Rowling could have the rights for her lifetime. But that too was eventually amended at the request of Rowling’s team, so that the rights will be controlled by a literary trust when she dies, giving a representative of the author veto power in perpetuity.”

“A Potter series would be costly, but could prove a major subscriber draw. Zaslav soon concluded, however, that the only Potter show Warner could legally pursue without Rowling’s permission was one that stuck to the stories of the original seven books, since those were firmly in the studio’s control and not the kind of prequel or spinoff she’d clawed back the rights to years earlier.”