Don't feel the need to explain K. Rool, or BWD. Or Takamaru. Or Rex. Or Spring Man, really.
Ashley is popular, and is set to be in WarioWare Gold.
Kamek makes important appearances in both Yoshi and Super Mario games.
Tropical Freeze port makes Dixie Kong relevant. She was already going to be in Brawl, and she would help double the number of characters DK has now.
Decidueye really isn't popular in Japan. Don't think Nihilego is any more popular, but it is important to Alola's plot, even being connected to one of the characters, and it would represent the new group of Pokemon. That said, I could easily see it replaced with Mimikyu.
Villager opened the door for more Animal Crossing characters, even though it isn't a fighting series. Isabelle is supposedly preparing to be one of Nintendo's more important characters. She's been consistently important since her debut.
Savvy Stylist would be the game's
; I remember a bunch of people saying it'd be Badge Bunny, which makes me think it'd be someone else for an actual surprise. Her series actually received its fourth game in November 2017.
Dillon is already getting his third game, meaning his series is already a mainstay.
Chorus Kids were apparently going to be added to Sm4sh. If that's true, that makes them a lot likelier for the Switch, especially thanks to Megamix.
Crash and Rayman are gaming icons. The former also had his games ported to the Switch.