You can be good at something you're qualified for, but greatness most often comes because of people doing something they're unqualified for. How can that be?
Ever hear someone say "he's unqualified for that"? How do you become "qualified for that"? I became qualified to call myself an architect by attending five years of school at a university accredited to teach architecture. I then worked three years under the watchful eye of NCARB's IDP system in order to become qualified to take the NCARB exam... which I finally passed on my third try. Actually, I passed all but one part on the first try, but that's another story... Until I passed the whole thing, I wasn't qualified to be called an architect.
So you become qualified by learning a recognized discipline and then testing your knowledge to the satisfaction of the testing body. But let's think for a minute. People who change the world aren't usually doing something you can go to school and learn... they're more likely inventing things, or inventing different ways of doing known things, that haven't even occurred to anyone else yet. In other words, they're doing things they're completely unqualified for... because there are no qualifications yet. The university courses and the regulating bodies come along years later because of the work they're doing now.
But here's a funny thing... when you do something you're completely unqualified for and it turns out to be something quite useful, then you do it again and again. When you've gained a basic competence in the thing you've invented, then at that point you go from being unqualified to being uniquely qualified... because you're the only one that can do it yet.
Here's another thing to consider: If you want to become the best at a known discipline, then you have a nearly impossible road ahead of you. Wanting to become the world's best attorney, the world's best physicist, the best quarterback in the NFL or whatever pits you against large numbers of outstanding intellect, talent, and experience. So that's likely to be a pipe dream. But becoming the best at a discipline you invent is... well, it's pretty much a sure bet, at least at the beginning. The key is this: is the thing you've invented highly useful? If not, then nobody's going to care. But if it is highly useful, you just might change the world.
Here's another thing to think about: Becoming uniquely qualified to do something extremely useful is likely to make your time doing that thing very valuable because you have no competition. That means you likely will have more spare time to do other things you're unqualified for, and maybe discover more useful things. It's a virtuous cycle. I wrote about my own experience with this virtuous cycle in the last half of this post, if you're interested.
So go do something you're completely unqualified for!