Dragon Ball made it clear that Gohan surpassed his father in terms of strength and always had the potential to do so. The inevitability of the next generation surpassing their parents also applies to Gohan and may explain where Dragon Ball is next.
It has long been suggested that Gohan had the ability to become even stronger than Goku, almost from the moment of his introduction into the series. Gohan quickly mastered advanced techniques at a younger age than Goku, and had his latent potential revealed several times throughout the series. He has unlocked unique transformations such as Ultimate Gohan and his latest, Beast Gohan, which have helped him stand out even more. There were always every sign that Gohan was not only surpassing his father, but that he was always destined to be.
Gohan is the strongest for a reason
Gohan's strength provides a “passing the torch” moment
In a pamphlet distributed in Dragon Ball Super: Super Hero screenings in Japan, Toriyama was quoted as saying that Gohan is arguably the strongest character in the series, despite his lack of training in recent years. Gohan's strength has always been exceptional, and it says a lot about his power that even without training, he managed to surpass his father's strength. Gohan has the greatest potential of all the characters in the series, and that's no coincidence. In fact, it's essential to his character.
Gohan is introduced as a young man, mirroring Goku's introduction as a young protagonist in the original. Dragon Ball. The idea from the beginning was clear: that Gohan was the heir to Goku's power. He went through hellish training to prepare him for the coming of the Saiyans and developed his abilities quickly. Gohan is the representative of the next generation of heroes and slowly takes his place as Earth's hero. Goku's death further emphasizes the fact that Gohan is taking control of him, with his request to Piccolo to train Gohan and prepare him for what is to come.
This type of “passing of the torch” moment is common in Shonen anime and manga; My Hero Academy may do this in the most literal sense, by having All Might actually pass his own power on to Midoriya, but it's common in other series as well, especially in sequels (which Dragon Ball Z without a doubt it is). In a way, Dragon Ball Z it was supposed to be something like Boruto or Yashahimewhere a new group of heroes, mostly descendants of the originals, must band together to defend the world in their parents' place. Goku's popularity prevented this from happening directly, but that was still the intention.
Gohan must pass the torch alone
The time may be coming to embrace Pan's destiny
Gohan probably never fully accepted his new role as main hero, preferring to leave that to his father most of the time. But now that Gohan has officially surpassed Goku in strength, he has no choice but to take center stage. Gohan's power is simply essential to Earth's defense at this time, so he must accept the role. With this, the son finally ascended to his father's level, and the next generation becomes not Gohan's, but his daughter, Pan.
There's reason to think that, after Gohan, Pan might be the character with the most potential. Pan has been suggested as a prodigy, shown in Dragon Ball Super being able to fly as a baby and being able to accompany Piccolo in training, despite being a preschooler. Pan is still young, of course, but the potential to be great is present within her, just as it was with Gohan. This is, of course, despite the fact that she is only a fourth Saiyan, which suggests that her potential was inherited directly from her father.
Pan becoming the new hero in GT It was a great idea, but GT didn't let Pan flex his muscles as much as he probably should have. Pan never manages to become Super Saiyan GTfor example, but there is no reason to think she is incapable of it. In fact, Pan may have access to greater forms, like Gohan did, and that's worth exploring. After all, being half Saiyan didn't affect Trunks or Gohan's ability to get stronger, so why should Pan be limited by the fact that she's only a quarter Saiyan?
Gohan doesn't need to go anywhere
Gohan can pass the torch and stay relevant
It might seem like a shame to have Gohan outplayed now that he's finally eclipsed Goku, but that doesn't mean Gohan needs to die or go away. Gohan was able to continue to grow and become stronger while Goku was still around, so there's no reason why the same couldn't be true for Pan. In fact, it could be very interesting to see Gohan instructing his daughter and helping her to grow stronger in a way that Gohan rarely had the opportunity to do with Goku.
There's also the fact that Pan still has some growing up to do; at your current age, from Superheroshe's still too young to actually fight, even younger than Gohan was when he started. She may still be preparing and training for that day, just as Piccolo was shown training her in the film. Pan seems to take to training even better than Gohan, perhaps signaling his own latent potential.
In the end, passing the torch from Goku to Gohan and his generation was a major theme within Dragon Balland it is only right that this theme continues into the next generation as well. Characters like Pan deserve their own time in the spotlight and have the potential to be great warriors themselves. Gohan may be the strongest character in the world Dragon Ball franchise now, but it doesn't have to be this way forever, and there's no better choice than Pan to be the next hero.