It was simple... At the end of Dragon Ball, we had two possibilities:
- one was the strongest character in the entire universe(I do not count Gohan since he lost a lot of his power after Buu since he did not train, same with Goten or Trunks): Uub. He is human, he is a naive child, he starts as a "weak" one despite having access to the Majin's power, + his master is none other than the original main character. (we could almost say he was choosed to be Goku's successor as the Earth "defender", ultimately becoming permanently stronger than a mere limited in time SSJ3).
- the other was Pan. Potential first female Saiyajin protagonist. Like Goku in his youth, she's a lot stronger than the average people (problem with her is that she is actually stronger than almost anything) and she likes fighting. (+her parents had a former interest for martial arts)
As for the story, it should have been a return to the Dragon Ball part of Dragon Ball (like GT tried after the androids and Buu's sagas lost tracks of the DB).
The problem for a sequel is more about the opposite side. What could have been a good villain after a landless king, a megalomaniac's private army, a demon who wanted to rule the world, a galactic dictator whose only left desire was immortality, a stupid "robot/clone" who had absolutely no goal except to achieve its completion and then tried to destroy the Earth because "why not?" and finally some mindless slave god-slayer / planetes destroyer artificial evil djinn.
You can't just go on for more and more when you already reached a pic. (obviously in power AND brainlessness)
edit: actually, we can see the villain problem through the cinema trilogy. Beers is not a villain but a deity who does what it wants while the second movie simply recycled a former antagonist for a revenge plot.
