Then again, I think the same is true of Superpowers and Mutations in DC/Marvel, just look at how many people with powers and mutations in that have powers and mutations that suit their given birth names or the way they think right down to the ground like.
I know, I noticed that too. It's amazing…almost like their lives were a story, that someone was writing.
On a more serious note, let's take a look here. Spiderman got his powers by genetic combination with a spider. He got spider powers because it was a spider. It had nothing do do with him. The Flash got his molecules accelerated by lightning. The Martian Manhunter had no super powers, just a list of abiliteis that seemed like powers to us which were in fact merely evolved abilities of his race. Superman gets his powers from our sun, and in fact most of his powers (especially the original, before he could fly and it was just super-jumping) are in fact things which are aided or controlled by chemicles our body's partially create using sunlight.
You are up against a fanatic here, i could go on all day. The point I am making is that superheros, as a rule, get their powers in a way that makes it very clear what their power will be.
Now, One Piece and X-Men could be different. Control of the power would affect brain chemistry, which could in turn affect personality, and for One Piece the mechanism of the DF's is unknown. But your grotesque misconception that superheros in general (superheros like Captain Atom–his atomic structure specifically altered to become an armor-like shell and give him assualt capability by the US Army. And while we're at it, when they tried the same process on a different guy, they got the same results, albeit Major Force is purple where Captain Atom is silver) have powers not dictated by the way in which they gain them.
And just to prove I'm a real loser, if your knowlege of the DC universe is as thorough as mine you'd know that (another example: Mathew Blood and Etrigan: Mathew Blood turns into Etrigan because he serves as Etrigan's prison) in the Invasion multi series mega saga, they explained the mechanism by which superpowers are gained: Some people have a gene in their bodies that, when they are put in mortal danger, instead adapts to survive and grant extra abilities to prevent it in the future. The manner of that mortal danger affects how the powers function. Thus, you see, the way in which DC superheros get their powers is in fact officialy a control of what powers they get.