There have been a lot of proposed additions to USF that didn't see the light of day, or were built elsewhere in the resort. While that does give us insight to what type of lands and IP that have been considered for adding to USF, its also important to consider that these "what-ifs" are not the direction the park ultimately took.
If we had never gotten a second and eventually a third park, USF likely would've looked drastically different. It likely would've been much more of a random assortment of various IP lands with no real correlation, much like the other "Universal Studios" parks around the world that never got a second gate. Jurassic Park could've ended up at USF, but they ultimately decided it would be better suited for IOA where it could be better fleshed out, and serve as the park's anchor. KidZone was really the only place Super Nintendo World would've made sense space-wise in the original resort before Epic got the greenlight, and even if it was built there, it would've been self-contained much like Diagon.
I'm somewhat skeptical that a Diagon-style transition is possible in the Simpsons plot, and even if it is, that probably would not be the best utilization of the land given its shape. It's also worth considering that given the success of Wicked: For Good, they will likely want to make a land that is "fully immersive" similar to The Wizarding World and the lands within Epic. I don't think they are going to accomplish that if they use Simpsons, as it would not only be visible from the whole lagoon, but the surrounding park would be fully visible from within the land.