Here's the thing with 3rd Party characters...3rd party characters include characters that come from companies/studios that aren't gaming as well.
Super Smash Bros. is advertised as a gaming crossover brawler. Even the E3 direct took a moment to cover the fact that this is "the biggest crossover in gaming history." The Youtube page says, "Legendary game worlds and fighters collide in Super Smash Bros. Ultimate!"
Not only would an anime, cartoon, or non-gaming character be shocking, it would literally go against the design of the game and series itself. Adding Goku, Mickey Mouse, or any other non-gaming character goes against that "biggest gaming crossover" vibe.
Sure, compare Smash to Soul Calibur, Tekken, or Mortal Kombat. They're not comparable aside from the fact that they're fighting games with guest characters. They're completely different in concept. Including Negan in Tekken, a game of completely original characters, makes him a guest character. Negan doesn't go against the theme of the series itself. It's a terrible comparison.
This is the same with Freddy Krueger in MK9, Yoda and Darth Vader in Soul Calibur, or any other guest character in a given game.
Basically, the theme of Smash is, "Gaming titans collide". Adding Goku literally goes against that theme, and therefore the direction of the game and series. Not only has Sakurai hinted that Mii Fighters are the solution to having non-gaming characters in Smash, but he's said they're impossible". As a game developer with a sense of professionalism, why in the world would he go back on that of all things?
I've literally yet to see a compelling point as to why Goku or another non-gaming character would be added. Sure, the responses are, "Sakurai said Ridley was impossible, and look where we are". True. It doesn't take much critical thinking to understand it's a completely different ball game though.
It's not impossible. It's illogical. It would literally be like adding Master Chief to Playstation All-Stars Battle Royale. It's not impossible. Sony could have if they wanted. But it would have literally gone against the design of the game.
Move onto another topic. PLEASE. THIS IS BORING AND A WASTE.