10(Same) Another Set of Dragon Balls
One of the most important plot points in Dragon Ball Z is the revelation of another set of Dragon Balls, more powerful than the ones on Earth. It reveals that the earth set, while important, are not unique.
On the other hand, the Black Star Dragon Balls are the key that
kick off the plot to Dragon Ball GT. The Black Star Dragon Balls are spread throughout the galaxy. They're a precursor to the more well-known ones used throughout the series. They turn Goku into a child, and, if not gathered together again within a year, will blow up Earth.
Like the prior set, this develops the lore, further expands on the universe, and returns the focus back on collecting the titular Dragon Balls.