Not much of a topic, but I'll bite.
I believe if you do a little research you'll find that (or at the very least it is theorized that) Toriyama intended to replace Goku with Goten after Cell exploded and killed him. Goku was too popular though, so ultimately he was brought back. That said, Goku isn't necessarily always the "star" of the show. There were several gaps where Goku was barely seen, most notably when he was dead and Gohan became the focus, though in fairness as soon as Buu showed up Goku came back. Gohan didn't have a major enemy to fight.
Some sort of off-shoot focusing on Vegeta would be cool, maybe a short series focusing on his time working for Frieza. I can think of a few ways they could make that interesting. Making him the main character of the main series though? That just doesn't work. He's a darker counterpart to the main character, and in a lot of ways he's there to balance the main character out, not replace him.
Oh, and there were only a few occasions when Goku and Vegeta were more or less equal in power, and each time Goku soon jumped ahead again. That was kind of the whole point.