If they wanted to return to the Zelda well, I think there's a fairly clear place in TOTK's story to insert the game. However, with the Xenoblade Klaus trilogy wrapped up, I think now would be a good time to do a Xenoblade Warriors game, potentially with some XBCX content or DLC to promote a remaster. The main challenge I see is that Xenoblade doesn't sell as well as Zelda or even Fire Emblem (I think), so perhaps they'd want to go with an IP with a better sales record. Mario, Pokemon, Splatoon, or Animal Crossing (how? no idea), maybe. I'd definitely be on board with Kirby as others have mentioned, and I wouldn't be shocked if we got a Pikmin Warriors title if Miyamoto likes the musou games.
Ideally, I think what would appeal to me is a Nintendo Warriors game that focuses on Nintendo's smaller IP and less famous characters. I'd love a roster that has Chibi-Robo, Kid Icarus, Helirin from Kuru Kuru Kururin, Sami (or someone) from Advance Wars, Mr. Game & Watch, Jenna or Mia from Golden Sun, DK Jr., Waluigi, the Prince of Sablé from The Frog For Whom the Bell Tolls, Takamaru from The Mysterious Murasame Castle, and a random assortment of characters I barely know. I guess it'd be like the Warriors equivalent of Captain Rainbow to a degree. I think the villain would need to be someone from very early in Nintendo's history that I am potentially not aware of, kind of like how Epic Mickey brought Oswald back. Original DK or maybe even "Jumpman" might be fun, with the twist that Jumpman is an impostor of some kind like Shadow Mario in Super Mario Sunshine and then you unlock Mario upon beating the game.