Hey, who doesn't want to be the best writer in the world? Unlike the rest of us schmucks, though, John Keats is fairly certain that he can be the best there's ever been – provided, of course, that he lives long enough to see his poems published. Ironically, the fears he voiced in "When I have fears that I may cease to be" were realized (he died before his poems were published), which makes this melodramatic assurance of his own failure far less silly than it might otherwise seem.
Just as the speaker seeks an impossible love, his ambition for achieving his literary goals are also unattainable. No artist can ever possibly realize all of his ideas before he dies.