I'm going to let some thoughts here..and expect it to be a doozy.
So if it seems that Diagon and Nintendo are definite then that raises a big point; for the next two major Universal Hollywood projects, we're talking DENSE areas.
I've made a form of what i'd think could be where they're going and i'll explain.
Now let me explain the obvious, whats with nothing at the Mummy plot except a plaza, new London park and a new road for guests? Well I think once Mummy closes (which should, "Should" close after the first summer of Nintendo's lifecycle here, we'd see Mummy announced to close right after Labor Day weekend of that summer. But I think Deconstruction for Diagon will be even before that, with the soundstages starting behind Mummy.
What replaces Mummy is somewhat immediate for the year after the closure, a new plaza and relocated Wheelchair disabled pick/up area next to the future area of Diagon Alley. The Plaza could be a form of filler in-case of Diagons overflow but it'd also allow it to reach the Jurassic Park side with the new over-flow road; shown in the photo.
The Plaza might also act similar to how Universal Plaza works with a new Meet and Greet for the Transformers characters located there. It'd work in a way that it'd allow new footspace for guest flow and actually good for when Diagon needs line space for TMs to easily get the line done.
Now for the case of Diagon itself..It'd have a expanded building for Magical Menageries, the beverage shop in there; and more importantly, storage for certain merchandise. Now, the way I see it also, Diagon has to have a Ollivanders. The way it'd work is more just for getting a more equal amount for the Hogsmeade version and the Diagon version, (it'd have the room count it has in Orlando).
And the obvious, It'd have no Hogwarts Express. As much as I hate that it'd not work for this version, it makes sense from a size point of view and layout point of view.
It'd also allow another backstage entrance or what i'd think is probable, the Transformers Exit being re-formatted with a brand new façade. It'd be a win-win for A&D and the fan base to fix the façade and entrance. Now, it'd also possibly be the loss of the Transformers store.. (which I see something happening to the Character shop..for this..)
One final thing, Nintendo. Upon looking at it, I'm leaning to where Sean from Parkscope said on OU. if this will be anywhere similar to Orlando's counterpart, it is going to have to be a DENSE area. And from what it seems as well; five attractions for that version. So the way I see this entire Nintendo area, it has to be more on the format of Dense and Quantitative for the attraction, even though it'll still be at a high amount of quality.
So, with that said I have this sort of timeline..
2018: Nintendo (Launch with Orlando very similar to the Simpsons Ride launch where it'll be few weeks off)
2019: New Lower Lot Plaza/Wheelchair location/Road
Late 2020/Early 2021: Diagon Alley with new Transformers façade/Exit/Visual enhancements.
Now, I can be either correct or extremely off but I think this is where Universal would WANT to go, but that's just my thinking.