Before Super Nintendo World decided on its Mario theme, there was apparently some discussion within the Japanese video game giant and Universal Studios about "other" Nintendo-themed IP being incorporated into the park.
Speaking to Polygon, Nintendo's Shigeru Miyamoto mentioned how Splatoon was one of the series considered outside of the company's key franchise, Super Mario. Nintendo instead decided to make this new venture "easier" by narrowing focus on its lead mascot:
Shigeru Miyamoto: “There was discussion about adding other IPs in there, like Splatoon. But instead of having our focus scattered, we thought it would be easier to start with something focused. That’s why we ended up starting with Mario."
Although the focus is mostly on the Super Mario series right now, Miyamoto also teased "some other IP" sprinkled throughout Super Nintendo World in the backdrop. Pikmin is one of the more notable sightings so far.
Nintendo has previously announced it will expand the park with a Donkey Kong expansion as well, set to open in 2024. Considering DK is also in the upcoming Super Mario Movie, and is one of the oldest Nintendo characters, this makes a lot of sense. This particular part of the park will allow guests to walk through "lush" jungles where Donkey Kong and his friends live. There will also be a roller coaster, interactive experiences, and lots of themed merchandise and food.