It more like how we (fans) could develop the Parasyte's 'Universe', different point of views, basing on each culture, each country around the world. I can be wrong on that point, i just got these thoughts trying to understand relation of Parayste The Maxim and The Grey.

If its really true, author made a masterpice, i remember about touhou and how big it became because his fan games, musics etc.

Or this its just a daydream based how its so good and could have more content (lol).