Toriyama originally intended to leave Goku dead after the Cell Saga and make Gohan the main character from then on out. He only brought Goku back because the Japanese fans were whining.
Honestly though, he pretty much admitted that he made it up as he went along. He originally wanted to end the series after the Freeza Saga, but it was too popular.
Then, he originally intended for Androids 19 and 20 (the fat one and Dr. Gero) to be the main villains of the Android Saga, but his editor thought they were lame and wanted him to make some better villains. So then he created 17 and 18, but his editor still thought they were lame since they just looked like regular people and weren't threatening. After that he created Cell, so 3rd time's the charm.
But I guess the point here is that he really didn't plan the series out at all, he was making it up chapter by chapter. And I'm not hating, he pretty much admitted this.
Toriyama brought back Goku to the living because his fans were mailing death threats. Yea, actual death threats.
And it was clear that post-Freiza DBZ, Toriyama was pretty much lost in where he wanted to take the direction of the series, at times. He had a plan for everything in the entire Dragonball franchise, until then.
Piccolo was probably strong as Perfect Cell in the Buu saga, I assume. I know that Dende said that he kept on training after the Cell games.
But Majin Vegeta did outclass Goku.