If it was just Universal Creative making the decisions I'm sure we would see Nintendo's three biggest properties equally distributed across the parks (Mario at Epic, Zelda at IOA, and Pokemon at Studios). Luigi's Mansion is a perfect fit for the space inbetween Super Nintendo World and Monsters. Mario Galaxy matches the aesthetic of Celestial Park. But Universal has to make these decisions in conjunction with Nintendo, and Nintendo is a very very odd company.
In an interview with Miyamoto on why Super Nintendo World only includes Mario characters he said "There was discussion about adding other IP like Splatoon, but instead of being scattered, we thought it would be best to have something focused and make something that is quality and good. That's why we ended up starting with Mario."
So while Zelda is the obvious pick for IOA, I wouldn't be surprised if Lost Continent sticks around and instead we end up with Splatoon Lagoon and Epic gets Zelda instead. I think anything is possible at this point.