People seem to be hinting it's The Great Hall restaurant, though there's feasibly enough space back there for a smaller ride, a restaurant, and other experiences.
Kind of a wackadoodle system though if you need to go to Epic Universe to access The Great Hall. You don't go to Hogsmeade for the Great Hall restaurant and you don't go to London for the Ministry of Magic ride. You'd have to transfer to those places from Paris, like it's the Wizarding World version of the Denver airport.