I'd say Splatoon is definitely the best new IP that's not the two main ones. Though it's just too narrow of a genre to have mainstream appeal. What I could see is a massive expansion of the Splatoon universe into way more games and other genres. I quite like the whole Splatoon aesthetic and the world but I'll be damned before I play a shooter on a console without keyboard and mouse. A platformer, action adventure or even RPG would be neat.
Uh, not mainstream enough? Splatoon 2 has outsold every 3D Zelda release prior to BotW and every 3D Mario release prior to Odyssey and is still going. I would call that pretty darn mainstream. Arms is their niche creation.