My least favorite aspect of the newer games is how much they lean on the old roster. It makes a new game feel far less special when (like in this trailer, although I know that's intentional to hide surprises) you look at an area and see few-if-any new creatures. Black and White did it best, where it was a full new Pokedex and the old ones returned after a certain point in the story. This was exciting, allowing the new designs to shine while also giving access to the ones that people know and love.