Decided to stew a bit more on the subject--and let's say, if Springfield goes for London and Diagon...how does that not include the Studio Tour plaza.
If Gringotts comes to USH, it would be a 2.22 acre attraction, that couldn't fit into The Simpsons Ride without heavy changes to roadways, infastructure, and what-not.
However, there is one area that could work rather well for this potential option: The current spot of where the maintenance shed goes. If Diagon would come, Gringotts would need land; about 2.22 acres worth. Hypothetically, they could do something really well with that of the area at the maintenance barn, alongside a small portion of the plaza. And then for the rest of Diagon, have it within the plaza and touching a small portion of the hillside, with a connector over where the current Tram entrance is.
Now with that said, how would you tackle the issue of the elevation of the tram plaza? Simple: you don't. You have Diagon be elevated to be above the height of the current tram plaza as it stands now. Essentially, think like a mall. That would allow the area to blend in a bit more with that of a new London (in the wake of Springfield being turned into that of London), and saving The Simpsons Ride and Special Effects Stage down the road, with both the VR attraction and the Ministry of Magic attraction from Epic Universe.
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Simpsons is a weird anomaly to me when it comes down to it. One that I can't see much coming, unless they get super duper creative with the footprint of The Simpsons Ride.