When the Studio Tour first opened, the intention was that it would showcase HP stuff for a few years until interest waned, then pivot to showcasing costumes, sets and props from whatever the next popular franchise they had was. That's why it's officially called the Warner Brothers Studio Tour: The Making of Harry Potter, not The Harry Potter Studio Tour, despite what people might refer to it as. The fact that they never pivoted away and instead chose to create new areas, themed food spaces, and expand their footprint goes to show I think that they've decided HP is there for the long term, not the short term.
I wouldn't be at all surprised if their next expansion included a dark ride. They already have green screen photo experiences, immersive events (like after hours dining in the great hall) and pseudo-ride experiences (like on the train). I don't know how much space they have to work with on the site, but adding a theatre show or a ride experience of some kind seems the logical next move for them.