One of the most popular fan requests for a character addition to the Super Smash Bros. series is Goku from the Dragon Ball franchise, but he likely will never make it into the game as long as Masahiro Sakurai is in charge.
Masahiro Sakurai is the creator of the Super Smash Bros. series, but he also writes columns for Famitsu magazine. Sakurai recently released a book that compiled his articles from 2015 to 2019, which included new content related to the development of Super Smash Bros. Ultimate.
A user on the Smash Boards forum named Ayumi Tachibana has translated parts of Sakurai's latest book and posted them online. It seems that Sakurai actually addressed the notion of non-video game characters and named both Iron Man and Goku as characters who are unlikely to appear in the series.
Goku is one of the most popular anime characters of all time and has starred in numerous video games over the years. However, he debuted in manga and is best-known for being a cartoon character, which is one of the main reasons why he isn't likely to appear in the Super Smash Bros. series. There are some characters in Super Smash Bros. that tow the line when it comes to being strictly video game representatives (most notably R.O.B., who is a toy), but Goku might be pushing it. The same thinking also applies to Shrek.
Sakurai also made some other interesting comments in his book, such as how he was allowed to play the beta of The Legend of Zelda: Breath of the Wild before anyone else, as he had to know how Link worked in the game. Sakurai said that the idea for a mode similar to World of Light was considered since Super Smash Bros. Brawl and that he's not sure if he would include Spirits again in the future. The concept of character balance was also brought up, as Sakurai noticed that the online win ratio for all of the cast is close, so he is unsure if more changes will be needed.
It seems as if Goku won't be making his way into the Super Smash Bros. series, but Sakurai won't be in charge forever and it's possible that a future director might make a push to bring the Dragon Ball hero into the series.