Nintendo have done a fairly good job in trying to fit Arceus into the whole mythology and legend of the Pokémon world. Mew was never regarded as the creator - only as an ancestor. To compare it to the Abrahamic religious traditions, Arceus is God, while Mew is Adam/Eve. What Nintendo have done, in my opinion, is write themselves into a corner, as it were. We've always had some nondescript legendary Pokémon such as Mewtwo, Celebi etc. that don't really have too much bearing on the legend of the Pokémon world, but there have been those with various powers that affect the environment. Subsequently, Nintendo have created more powerful abilities, as it were. The Legendary Birds had control over the elements, for example, but as time's gone on we've seen 'guardians' of the skies and seas (Ho-Oh and Lugia respectively), Pokémon that control the climate (Groudon, Kyogre, Rayquaza) and Pokémon that control time and space themselves (Dialga and Palkia). Where does one go after controlling the fabric of spacetime? Logically, one could argue, we move to deities. With Arceus Nintendo have created an ultimate god - one closer to the Abrahamic God than the kami of Japanese Buddhist tradition. I can't see anywhere to go from here that would continue the tradition of a new set of legendaries being more powerful than the last.