Goku faced threats beyond modern-day Earth, including Frieza and Cell, who represent space and time challenges. Frieza inspired by land developers, was symbolic of space, while Cell symbolized the future. Victory over Frieza and Cell solidified Goku as the most powerful mortal, leading to Dragon Ball Super.

Dragon Ball chronicled Goku's rise as the strongest being on planet Earth, and Toriyama felt it was only natural that the scope of his threats began to expand beyond Earth. Toriyama's reasoning for how he came up with Frieza and Cell provides some insight into how the two iconic villains came to be, and how they're both secretly connected by this theory of challenging Goku.

It was only with the advent of the Dragon Ball Z era that the series began to see threats emerge from beyond Earth, and the franchise began to transition to being more sci-fi than fantasy (although fantastical elements...