While I am a long-time fan of the Kirby franchise, I'm not sure how well it would translate to a permanent theme park experience. It doesn't have that many recognizable landmarks compared to Nintendo's other major franchises, and I can't think of any ride system that is well-suited to an attraction based on the games.
As for Splatoon, I don't think it's currently popular enough in the United States to warrant having an attraction here. While the popularity of the series has continued to exponentially grow in Japan, I feel it's popularity in America peaked with the launch of the Switch/Splatoon 2, and it has gradually become somewhat more niche in the years since. I think it could work very well as a land with USJ though.