Argument 1
If there are keybladers who can transform their keyblades into all sorts of things, example being the keyriders...
And the law of conservation of mass doesn't seem to apply, again example being TVA's rides...
Then that means that, given enough potential, keyblades can possibly be turned into planet-sized structures.
Argument 2
If argument 1 is true...
And almost anything large enough to be traversed on can be considered a world in KH, example being Mostro...
Then that means that a keyblade "world" could theoretically exist.
Whether or not it can possess a heart is still up to debate, however. In the same vein, even if, story-wise, it is technically not a true world, gameplay-wise, it can still be considered one, in the essence that it is an in-game level.
Any thoughts?