If they expand Illumination any further, why wouldn't they just opt and take out French Street/Parisian Courtyard for more? I do not get why people keep thinking Illumination is going to expand out of the left side. Plus, it's as
@Freak mentioned:
Animal Actors, Castle Theater, Butterfat/Luigi/Duff Gardens; on the other hand, would have a far more likelier chance of being removed for one large investment due to it's geographical placement. I also think NY is out of the card, unless if Universal wants to have three completely different NewYork areas in one park (yes, they have two).
My bet is whatever SES/Animal Actors becomes, it'd become the anchor of a revamped Springfield. I do agree with Freak that TSR if it's prepared enough, could survive one more retheme. They'd probably have to upgrade more beefier elements with it though.
To give thoughts on plot sizes, this comes to mind IMO.
A would be able to house a show like Sing on Tour, near to it's size. It could even be extended a bit further into the hillside if need too, and if they need to expand the parking structure to increase it's weight.
B, is the far larger potentiality. It can house a Mummy level attraction rather well; and that's not without going closer to the road in the area itself. They could easily morph in Feature Presentation into the size, and it'd allow for more space. I'd be shocked if they wouldn't want to have a proper E-Ticket be the landmark of the land. More-so than anything.
Geographically, Castle Theater and Animal Actors have little options for growth. But I do not think the solution is splitting it in half, for Illumination and something else. My belief (And albeit hope) is that they would use it specifically for whatever happens to Springfield. Especially as outside of the cluster of soundstages next to Transformers, and the Studio Tour Plaza/Maintenance barn; this area would likely be the biggest potential chance to get another large attraction.
Heck, I don't even rule out what
@Freak said, but I'd argue that they would put that to Pokemon more than doing something with a generic New York.