Mario has the greatest reach of all the Nintendo franchises. They definitely need some Pokémon stuff too (as long as they can work something out with TPC). But other than that, what other Nintendo stuff do they need to get ASAP?
I realize Zelda and Animal Crossing have had a recent (unusual) surge. Zelda is definitely fit for a ride. Animal Crossing maybe a store or restaurant. What else do they have? Kirby maybe, but how many people actually play Kirby, versus those who know him from Smash. Star Fox, Metroid, Kid Icarus, F Zero, and Earthbound are all incredibly niche. Splatoon maybe?
Mario and Pokémon are the big two.
I don't think Kirby would have its own ride, but something like merch and meet and greets would be best. Zelda is definitely one of the most popular Nintendo franchises that does needs their own attractions. SNW itself does need a bit more variety like having a Zelda area for example to balance things out. If I have to guess, Zelda would probably the only non-Mario area in the park given USJ's limited space. Pokemon and Animal Crossing should cameo the area as well.