Dragon Ball Z and the franchise as a whole have been seen as primarily Goku's story, but that wasn't always going to be the case. It's well-known that the series was meant to segue into following Gohan instead of his father, though this ended up not happening. Perhaps the biggest missed opportunity to pass this torch was in Dragon Ball Z's ultimate saga, wherein Gohan was more hyped up than ever.
For much of the Buu Saga, it seemed as if Gohan was being set up to finally become the new protagonist. By not allowing him to even defeat Majin Buu, the franchise cemented the fact that it would likely never move past Goku and give his kids their deserved spotlight. This set Gohan up for his later life as a scholar, but allowing him to beat Buu could've changed all that.
The De-Evolution of Gohan Began In Dragon Ball Z's Last Saga
After the Cell Games Saga, it seemed as if Goku would truly fade into the background and allow his son, Gohan, to become the new protagonist. Examples of this include his being the main character in the Great Saiyaman saga, wherein he tried his hand at going to school and even acting as a sort of local superhero. Eventually, these stories would give way to the Buu Saga, and it seemed that it would be Gohan who took down Majin Buu's upgraded form Super Buu in the same way that his father once fought Frieza. To make it even more apparent that Gohan was supposedly the focus now, the anime had him and Super Buu strike similar poses to the stances Goku and Vegeta once took during a fight. Gohan even wore a version of Goku's costume to make the resemblance even stronger. Sadly, all this hype was for naught.
By the fight's end, Gohan was defeated by Super Buu and consumed in the same way as Goten and Majin Buu's more heroic and corpulent side. This was completely anticlimactic, especially after all the special training Gohan had received before the fight. From there, both Goku and Vegeta would return and be the ones who ultimately defeated Buu. This capped off Vegeta's development into a hero, but it robbed Gohan of his own development. It also set the stage for his becoming less important in Dragon Ball GT and Dragon Ball Super, with the latter seeing him become a scholar instead of being focused on training. This utterly ruined the character, but a different outcome for the Gohan/Super Buu fight would've cemented him as the new Goku.
Gohan Defeating Buu Is DBZ's Biggest Missed Opportunity
As mentioned, Gohan was being set up as Goku's replacement for a while by the time of the Buu Saga. Series creator Akira Toriyama had wanted this to be the case, but Goku's overwhelming popularity prevented this. Still, the character development and fulfillment of Gohan's potential were very evident in the later parts of Dragon Ball Z, with the events right before the Buu Saga suggesting that he could find a life balance that his oft-absent dad never did. He could both fight and pursue more academic options, which would fall in line with his father's legacy and his mother's wishes for him ever since he was a child. This might have made for a much different Dragon Ball Super, with the stories being based around a man trying his hardest to take care of both sides of his life. This could have also seen him go on adventures with Videl, undoing the mistake made by Chi-Chi's simply becoming an overbearing housewife after marrying Goku.
None of this happened, however, because Gohan didn't beat Buu. This made so much of his character development fall flat and feel pointless, especially since having Gohan kill Super Buu instead of prolonging the storyline with Kid Buu was an exercise in meandering. By DBZ's conclusion, Gohan would have been Earth's premiere defender like his father was before him, all while achieving greater power and learning techniques that Goku never did. Instead, he was merely a stepping stone for Goku and Vegeta to cross in order to grab more of the spotlight, and the effect was incredibly detrimental to Gohan. Though he's still helped out in fights in the sequel series, it's certain that his somewhat less prominent role was set in stone due to his not being the one to defeat Majin Buu.