The Dragon Ball saga spans decades worth of lore. As such, it's become a multi-generational adventure featuring not just some of the original core characters, but also their children and, more recently, even their children, too. Given this, it's easy to forget how far ahead into the future the series got towards its original end.
While Dragon Ball Super has since retconned it out of continuity, Dragon Ball GT was originally intended to be the grand finale of the Dragon Ball saga for Toei Animation. The final episode said good-bye to Shenron and Goku, cutting ahead a century following the events of GT.
It is here where we meet one of the future generations of Goku's family line: Goku Jr. Who is this boy? What was his purpose? Was there anything planned for this young character beyond GT? It's time to explain one of Dragon Ball's most obscure characters: Goku Jr.
THE LEGACY OF GOKU
Goku Jr. has two appearances in the franchise: the Dragon Ball GT television special, Dragon Ball GT: A Hero's Legacy, which aired between Episodes 41 and 42 of the Dragon Ball GT original television run. It offers us a peek into the far future, after the end of GT, where Pan -- Goku's granddaughter -- is now a grandmother training her great-great grandson, Goku Jr. All the other Z-Fighters have since died of natural causes.
Pan gets sick at some point, prompting Goku Jr. to find the Four-Star Dragon Ball he grew up hearing about in order to save her, believing that it alone can summon Shenron to grant him one wish: to save his great-great grandmother. This takes him to Mount Paozu, where, over a century and a half earlier, Grandpa Gohan raised Goku Jr..
This requires him to fight a legion of monsters alongside one school mate of his -- making it a typical bully-turned-friend sort of story -- pitting him against the Demon Lord Yao, a pig demon who strongly resembles Ganon from The Legend of Zelda. Yao, in many ways, serves as a parallel to the Demon King Piccolo, the last adversary Goku fought as a child. During the fight, Goku Jr. very easily transforms into a Super Saiyan.
Goku Jr.'s almost effortless transformation into a Super Saiyan is actually a subject of controversy among Dragon Ball fans. It's confirmed that, should an ancestor turn into a Super Saiyan, it's easier for the descendants to transform as well, which might explain why Goku Jr. has such an easy time transforming for the first time. At the same time, however, the official Daizenshuu (guide books), when explaining why Pan couldn't become a Super Saiyan, argued that no one with less than 1/4 of Saiyan blood could transform into a Super Saiyan. As these were published before Goku Jr. appeared, it makes his transformation a massive plot hole.
When he retrieves the Dragon Ball, Goku Jr. tries to summon the dragon, but fails. When nothing happens, the spirit of Goku instead arrives to talk to Goku Jr. to explain to him how the Dragon Balls work. After their pep talk, it turns out Goku Jr. didn't need the balls at all. Pan just recovers from her illness on her own, leaving Goku Jr. stronger as a result of the adventure.
THE PROBLEM WITH GOKU JR.
The final episode of Dragon Ball GT creates another plot hole around Goku Jr., specifically for the aforementioned sequence of events. Goku and Shenron, at the end of the series, depart for "a long time" because humanity has proven it cannot be trusted with the balls. This comes after the final arc of Dragon Ball GT, which features the gang of heroes facing off against the Shadow Dragons -- demonic entities that are created after the Dragon Balls are overused. Each wish fills the balls with negative energy.
Goku leaves with the balls and, apparently, never returns... Until 100 years later, when his spirit shows up the GT special. There are a few issues here. First off, Goku, in his child form, disappears with Shenron, seemingly fusing with the balls. Presumably, Goku grew up in the century apart, but did he grow up with the Dragon Balls? Is his spirit tied with the balls, which, presumably, is why Goku Jr. can only see him when he finds the new balls? When did the Dragon Balls return? After all the people who abused and overused the balls died?
These answers are never answered. Indeed, as we see in the flash forward at the end of the series, only Pan remembers the alliance of fighters that saved the world countless times before.
The final sequence of the final episode features Goku Jr. facing off against Vegeta Jr., presumably a descendant of Vegeta. Despite both characters being named after the iconic fighters of yesteryear and statues being erected of these heroes at the tournament, neither character has any real idea of what their namesake's did. Vegeta Jr.'s mother (who is either a descendant of Trunks or Bulla) is still running Capsule Corp, so that company remains highly successful even a century later.
And yet, when both Vegeta Jr. and Goku Jr. transform into Super Saiyans, neither really understand what the transformation even means. They just see it as "turning your hair yellow." The two fight, though we never see the conclusion.
Pan sees Goku -- the original Goku -- watching over the fight before disappearing.
GOKU JR.'S FUTURE
Whenever a new generation of characters are established in a franchise, it begs the question as to whether or not some kind of spinoff featuring them is/was ever in the works. In the case of Goku Jr., the answer is a resounding... No.
When Akira Toriyama started the Saiyan Saga, he created Gohan to serve as the "next generation" of the series. The finale of the Cell Saga, where Goku dies and Gohan defeats Cell, was supposed to serve as a passing of the torch event to cement this.
However, the Buu arc instead reestablished Goku as the core hero once more, while Dragon Ball GT went back even further, making Goku a child as a return to the days of Dragon Ball. This made any plans of continuing the story beyond Goku seem unlikely.
Toei produced multiple television specials showcasing side-stories around smaller characters on Dragon Ball's periphery. Most memorably, the Bardock and Future Trunks specials during Dragon Ball Z helped expand the world that the series otherwise would have been unable to explore. The special and finale should be seen as just that: offering fans a little dose of closure as the series came to its ultimate close. It was a closing vision of, not an introduction to, Dragon Ball's future.
Now, with Dragon Ball Super erasing Dragon Ball GT from the continuity, Goku Jr. can be looked back on as Goku's epilogue for that particular era.