A Shoo-In is a character that will definitely appear. Some call them shoe-ins, but those people are wrong and also idiots. The title is wrong kthxbai. It's obviously spelled shoo-in.
Little Mac - Confirmed
Palutena - Confirmed
Sakurai's Nipples - Leaked, and also hot
Chicken Little - Hinted at
Qwilfish - Has unanimous fanbase support
Ujadani - Relevant, popular villain rep from a revolutionary franchise
Sceptile - Since Gen III remakes were confirmed Sceptile is apparently a shoe-in because Pokémon can never have enough reps except when that includes Squirtle and Ivysaur.
Carmen Sandiego - Was confirmed by Sakurai before the game was announced.
Chrom - Because there's definitely a pattern here!
Chorus Men - Because there's a Sneaky Spirit in Smash Run and even though we haven't seen all of the enemies and there could be a different Rhythm Heaven rep it clearly means they're in.