There are these two small buildings in the back where the expansion plot is located that have exterior theming but hollowed on the inside and seem really old (One is named something like Kai Nui Kitchen, distinctly similar to the drop slide), I've always wondered what they were built for but seem like they would've been pop-up carts. An expansion would be helpful to distribute park times for a lot of the major attractions: Krakatau fills capacity by 2PM, Ko'okiri consisting pushes 100 minutes, Kala & Tai Nui reach similar times because Tai Nui hasn't been open since last December, and Ohyah / Ohno usually has an hour or longer. When would this happen? I have no idea: it's still a relatively new park and since it's a water park with limited competition (pretty sure Disney only has one of theirs open at a time now), Islands needs to replace TLC, Studios needs to replace Simpsons, Epic is still opening up and will see expansions soon, and there are a lot of ventures overseas that are being worked at all at the same time.
An expansion before 2030 would be nice and would certainly complete the park since I don't think there's any room outside of the small expansion plot anyway. Add in another coaster for novelty, a body slide to compete against the tamer Punga Racers after it lost its mat feature, and some unique third attraction to round the area off. Oh yeah, and redoing the lazy river cave to its grandiose concept art would make it a lot better to work in!