I predict that the Splatoon IP will become bigger than the Zelda IP. This isn't talking about lifetime sales for the series, it only refers to modern Nintendo and console / HH game VS console / HH game.
I also only mean bigger in terms of sales. For example, many consider Zelda to be bigger than Animal Crossing for the Nintendo brand. However, looking at sales, Animal Crossing has surpassed it.
It might not happen this gen, Zelda U may outsell Splatoon. I do think that Splatoon will grow and become bigger though, likely next gen.