According to reputable leakster Vergeben (via Nintendo Life), it seems that the upcoming game could feature Ridley from Metroid, Ice Climbers, and Castlevania’s Simon Belmont. It should be noted that Ice Climbers have appeared in previous incarnations of the game, such as Super Smash Bros Melee and Brawl, but skipped the Wii U version of the game, but it looks like it could be making a return for the Switch if the rumors are true.
However as Nintendo Life has pointed out, Ridley’s inclusion is questionable. This is because in the past, series head Masahiro Sakurai did not seem particularly positive about the inclusion of the character in the game, despite him being one of the more requested characters over the past few years.
In any case with Super Smash Bros being playable at E3 2018, hopefully we’ll have more details then. In the meantime Microsoft has stated that they wouldn’t mind seeing characters Banjo and Kazooie make an appearance in the game.