I am not one of those that spout about patterns in the games but it would look like 1, 3, and 5 introduced a lot of them as those were the where we got the most new Pokemon. 2 and 4 added pre-evolutions and evolutions to existing. I would be willing to bet that 6 will do the same as 2 and 4. We already know one with Sylveon. As for the stand-alones, 5 added a ton of them and there are still several from 3 that have not received a new stage.