As a big Splatoon fan myself, I would love to see it at the event, but I don't think there's a very good chance of it happening. While Splatoon's popularity has continued to grow in Japan, I feel like it's popularity has shrunken domestically in the years since Splatoon 2 released.
While it still has a very loyal niche following in America, Splatoon 3 fell pretty well short of the sales of the second game, and anecdotally, I just don't see people talking about it outside of its online community anymore. If another Nintendo IP is featured, I just can't imagine it would be Splatoon over other options, especially since it's never been featured at USJ for a special event so far.