CapitaineCrash
Smash Champion
I think Tabuu is loosely based on Marx from Kirby.If I'm not mistaken, Giygas is considered a universal threat? Maybe an OC can have a character that's kind of based on him.
Master Hand took elements from Wham Bam Rock and I wouldn't be surprised if Tabuu took elements from another character.
Giygas would be the perfect villain for a crossover event, so I say either him or a character based on him would be just right for the next Smash Bros game.