Ah, I remember the days when people wanted this…

Ridley
Pros: Major character in his underrepresented series, a villain- something Smash is lacking
Cons: Too big

K. Rool
Pros: A highly requested character who would be easy to implement and would fulfill his series's lacking representation and the villain role just like Ridley
Cons: Hasn't been relevant in about a decade

Bandana Dee
Pros: The 4th main member of the Kirby cast and the only one not to be playable in Smash so far and has a unique weapon in his spear
Cons: Might be viewed as a "minion" character where the bandana and spear aren't enough to set him apart from other Waddle Dees

Waluigi
Pros: A fan favorite with a lanky frame that would set him apart from the rest of the Mario
Cons: Sakurai doesn't usually focus on spin-offs in Smash and Waluigi has yet to make an appearance in the core Mario series

Bandana K. Ridluigi
Pros: The perfect size for fighting, a major character for multiple series (sometimes a villain but always a main character) some of which are severely lacking representation and others of which are so popular that they're bound to get a newcomer, always relevant, has appeared regularly in both main series and spin-off games, has a unique weapon, a fan favorite
Cons: n/a