I would like to know if there are other mangakas like Eiichiro Oda who make an illustration cover of a specific chapter to be like a side-story, and it's really like a canon story, even it just a side story, for example, this one:
Is this really common in manga industry? If yes, is there another mangaka that is making this kind of "unusual" side-story? And is Eiichiro Oda the one who started this kind of thing?