Outside of new Charms, I hope the following characters other than Peach and Toad, who I mentioned in a previous comment, get added to future updates:
Daisy: She could have gameply akin to her appearance in Super Mario Run.
Amy: She'd no longer be a costume for Sonic, and she'd have gameplay akin to her appearances in Sonic Origins Plus and Sonic Superstars.
Peppino: Ah, the Pizza Tower protagonist, who would be making the jump from Boll Tower to this game.
Angelo: Did you know that Mike Wiering (founder of Wiering Software) created a game akin to the 8-bit and 16-bit Super Mario games called Super Angelo? I recommend you check out the video by YouTuber, gruz (formerly killgruz), if you haven't.
Maria: Giana's Player 2 sister needs love, too.
Link: Don't think Mario's the only Nintendo mascot to enter the fray. Personally, his moveset would be similar to Zelda II: The Adventure of Link combined with his appearances in Smash 64 and Melee (as well as his Dash Attack in SSB4 and Ultimate based on the iconic OoT Jump Slash).
Samus: Another non-Mario Nintendo character who could use her Beams, Missiles, Morph Ball Bombs, and even suits. The Super Mushroom could giver her the Varia Suit, the Wave Beam, and the Super Missiles, the Feather would give her the Gravity Suit, the Space Jump, the Ice Beam, and the Ice Missiles, and the Fire Flower would give her a combination of the Varia and Gravity Suits, the Charge Beam combined with both the Spazer Beam and the Ice Beam (as well as the Plasma Beam), the Diffusion Missiles combined with the Ice Missiles, the Spider Ball, the Spring Ball, the Screw Attack, the Space Jump, and the Power Bomb.
Nabbit: The only TRUE unlockable character who is unable to use Power-Ups, but more than makes up for it by being unharmed by enemies and most hazards except lava, crushers, and bottomless pits (though if he touches an enemy, his invincible frames merely activate for a much shorter duration than most other characters).