A Dragon Ball ride. Perhaps as a re-theme of the now-dated Marvel Super Hero Island based on the franchise, if not Anime in general. I could see Dragon Ball being used for the Hulk coaster in particular.
A revival of Back to the Future: The Ride. With the Universal's contract for The Simpsons set to end around 2028, The Simpsons' Ride's days are numbered. That said, once the contract ends, if Universal chooses not to renew it, then the building will revert to its original Institute of Future Technology theme.
The land of Springfield that was built around the ride will also be re-themed to that of Hill Valley of the future. Attractions in the land will include:
The Cafe 80's, where guests can purchase a Pepsi Perfect along with any other future-inspired food and beverage items. The Cafe will also include a video arcade where guests can not only try their hand at "Wild Gunman", but also a myriad of other classic games from the 80's.
The Blast from the Past store, where guests can buy Back to the Future-themed merchandise, including replicas of the Grays Sports Almanac as well as the outfits worn by Doc and Marty in the different time periods they visited.
The Hill Valley Courthouse, complete with its famous clock tower.
A SpongeBob SquarePants dark ride. Surprising that the franchise has never had one, only flat rides, 4-D movies, and the occasional coaster.
A Trolls themed sing-along show, much like the Frozen sing-along show at Disney's Hollywood Studios. It's only a logical fit, as Trolls is a popular music-based animated franchise that's popular with young girls. The young girl market seems to be one that Universal's never really branched into before note No pun intended, and it could easily be just as popular with them as the princesses a few miles south. There's also the benefit of Universal owning Dreamworks Animation, meaning they won't have to pay any licensing fees to use the characters. A possible location could be in the A Day in the Park with Barney theater, as that franchise has long since faded into irrelevancy with it's target audience.
Barney has been confirmed as closed forever on February 3rd, 2021. With Universal confirming that their Epic Universe theme park is continuing construction, the idea of Super Nintendo World going into the space Kid Zone takes up is no longer valid. So perhaps the possibility of a Trolls attraction might not take much longer.