I mentioned it in the announcement-thread: a new Kid Icarus could be such a title - albeit impossible because guess what Sakurai was working on. Even though it got leaked, StarFox Grandprix could be that game, too. Especially if it's a meaty game with an exciting story-campaign and tight 60fps-racing. Other than that ... no idea. Personally, I'd love a new Endless Ocean.
Even a 2019 Kid Icarus announcement with an Uprising port to tide me over would put me over the moon (not that that would solve your no new games for 2018 problem). I'm normally not a fan of either linear shooters or score attack games, so I didn't expect to love KI:U as much as I did, but that game was just a riot. Multiplayer could use a second shot on a system that can display at better than 240p too.
And Endless Ocean... clunky mechanics and a nonsense story/characters couldn't take away from what was easily the most compelling chill gaming experience I've ever had.