Wario

Ability(s) added/affected: Shoulder dash, Jump range

Difficulty: Normal

Pros: Shoulder dash can kill enemys quick + does damage to bosses

Cons: Jump range is cut in half compared to Mario's

Likeliness: Low (Only platforms in his own games to date)

I could talk about every one of his Wario Land ability's, but I think this is a fan favourite. It can also work in both a 2D and 3D Mario game, and could be used to unlock alternative paths. Due to his weight he also can't jump as high as Mario can

Waluigi

Ability(s) added/affected: Jump range, Character control

Difficulty: Normal

Pros: If he charges his jump (like in Super Mario Bros. 2 USA) He can jump twice the height of Mario's normal jump

Cons: Similarly to Luigi, Waluigi would be slippery to control as well, 1.5x times as much as Luigi (just to even it out)

Likeliness: None (Nintendo hates this dude)

A fan-favourite, Waluigi would be a great addition to the game having a taller chargeable jump than all the characters. His traction does bring him down however, just to even him out between the rest of the characters