My earnest #1 prediction for a new Smash character is Ayumi Tachibana from Famicom Detective Club. She checks off all the boxes. First party so no licensing fees. Had a new game somewhat recently so her inclusion would double as marketing. Fills the obligatory "Nintendo history deep cut" addition that's been in every game since Melee. Moreover, she was planned as a character for Melee, and being a scrapped character from an earlier Smash game is the number one predictor of being included in a future Smash game.