The Mummy would probably get rethemed before going bye-bye. This is a park that can't put up a decent coaster because neighbors will complain, so they really can't get rid of one that doesn't bother the neighbors. It's a fun little coaster that just needs a better storyline, and an ending that makes sense!
As for the rest of the lot, namely Stages 28 and 22-25: I think it's gotta be Nintendo Land. The space being taken up by 22, 23 and 28 is about 350 feet wide. Gringotts is at least 375 feet wide. Even if they find a way to make it fit, the building would end right about where the food court/museum building starts, which means Diagon Alley would have to fit completely into the area taken up by the food court and 24/25. (Orienting the building the other way leaves an alley only about 100 feet wide between Gringotts and Transformers.) It COULD be done - without the Hogwarts Express and the "Piccadilly Circus" area out front - but it would require a whole lot of redesigning, which makes things more costly. I don't think they could snake the Express through a working studio, so that's basically why I don't think this will happen. That train is the breakout hit of Diagon Alley.
Nintendo is being planned for the KidZone in USF, and that area is way smaller than this area. Problem is, it depends on what that turns into. If ET goes, then were talking about areas pretty similar in size. If the lot between The Simpsons and MIB is included, then all bets are off. That lot is HUGE, larger than Diagon Alley WITH the train station! That lot, if used, would probably be home to some form of a Mario Kart racing track, though, which certainly won't be put down here unless it's indoors.
So my money is on Nintendo, mainly because they could open the two US lands simultaneously. There would also be several attractions - compared to Diagon Alley's one - and USH needs to up it's attraction count. It's also much easier to relocate attractions in differently-shaped areas than it is to redesign what is basically one large, weirdly-shaped building. (Diagon Alley could be plopped exactly as created - making it cheaper - in the Backlot and reached via Hogwarts Express, which is a good way of limiting crowds there.) Most importantly, the Lower Lot DESPERATELY needs something for the kiddies. And a Nintendo Land could be easily expanded if Soundstages 16-20 go away. I'd be a little tougher to do that with Diagon Alley.