Mario gets a complete moveset rework. He is now way faster and agiler in mid-air, with a moveset based on grabs and jump chains. Cappy is an integral part of his set, and "takes control" of enemies during the grabs. Mario can't really "control" the possessed foes, it's just a cosmetic way for them to be stunned. His Final Smash consists in him taking control of a big enemy from Odyssey, such as the dinosaur or the big snow clouds.
Link gets a visual and moveset rework. He is now in his Breath of the Wild form, and his four specials are the four Gifts of the Champions. He's way less item-based and relies more on his sword and his bow. No idea for a Final Smash.
Olimar loses Purple Pikmin and White Pikmin. His gameplay and moveset are more aggressive, with him actually going into fights, throwing punches and kicks and generally requiring him to go head-on into fights, as to fit his role of more experienced commander and survivor as opposed to Alphie, who's a more timid and passive fighter.
The Ice Climbers are finally back. Their kit is unchanged.
Mr. Game and Watch loses all its RNG elements from his moveset. Let's try to make him viable, m'kay Sakurai?
Little Mac actually gets a Final Smash that makes sense lmao.
Dr. Mario, Lucina, Toon Link and Dark Pit are straight up removed or turned into alternative costumes.
Duck Hunt is removed from the game. It's not like they had some flaws gameplay-wise or they were a bad character over all. I just hate the guts outta them lmao.
Lucas, Ganondorf and Falco are reworked more to be their own characters, since they have a playstyle different enough from their original versions to be, you know, actual characters. Best case scenario, Classic Ganondorf becomes a thing. Let the new one have his sword and his fancy hair, I want my Ganoncides and my Kicks of Doom.
The Mii Fighters gets more moves for their moveset and customizable Final Smashes.