If I could add any character into Super Smash Bros. Ultimate as DLC, I would choose Geno from the Super Mario RPG: Legend of the Seven Stars.
Reasons for Geno's Inclusion:
Cult Following: Geno has a dedicated fanbase that has long advocated for his inclusion in the Smash Bros. series. His absence has been felt since Super Smash Bros. Melee, where he was a highly requested character.
Unique Abilities: In Super Mario RPG, Geno possesses a variety of magical abilities and transformations that could translate into an interesting and versatile moveset. His ability to use different types of attacks, including ranged and magical ones, would make him a dynamic fighter.
Iconic Status: Despite being a character from a relatively older game, Geno is iconic within the Mario franchise and represents a unique part of its history. His design and lore offer great potential for creative stage designs and music tracks that could accompany his character.
Adding Geno would not only satisfy long-time fans but also enrich the game's roster with a character who has both nostalgia and unique gameplay mechanics.