For a far off FFL ride replacement, I could imagine a high-tech dark ride that essentially goes through some of the Potterverse's main locations in London. Given that Gringott's ride storyline stays within the actual walls of Gringott's, this second ride could explore the other main areas. Possibly utilizing trackless technology or even some future ride technology, the story could be that Dobby (realistically by this time, all main actors would be too old to return for filming) is leading you through various places to evade the death eaters. You load a ride vehicle in the Ministry of Magic. You tour the main lobby, the fireplace hall, the Department of Mysteries, and finally run into Dolores Umbridge whilst she is conducting a hearing. Something happens to provoke her, and she sends the Ministry officers and Dementors to chase you. Dobby apparates you away to 12 Grimmauld Place (a safe house) where Death Eaters already are and chase you out of the house into the street. Dobby then apparates you to Diagon Alley. Arriving at the upper level of the Leaky Cualdron, where a Death Eater is keeping watch, Dobby immediately begins to fight him. Chaos ensues that throws you out of a window and down into Diagon Alley. Dobby shows back up at the last minute to save you, and returns you to safety. Easter eggs throughout the ride could include seeing the Knight Bus speed by as you apparate or possibly showing Wool's Orphanage.
That's pretty much what I think could be a pretty good ride that explores some neat places for a Phase III attraction.