Pokemon Sun and Moon improves the game by allowing possible tapping into new dimensions and realities of the game, and it brings in references to more characters along with more chances to capture and train rare Pokemon. It's also showing promise for more Pokemon battles between the three different teams.
Although the addition of a Snap feature in Pokemon Sun and Moon is a slight improvement, it hits the right buttons for a certain audience segment that loved Pokemon Snap. Overall, I don't think the feature will interact with the gameplay very much, but these days games like a social aspect as well, and "Snapping" your pokemon will result in some cool social media posts.
Please understand that I am not condemning the feature, it sounds quite cute and fun and is a nice throwback but it is not, by definition, an improvement on the gameplay or the quality. It is simply a nice extra feature to have fun with that may enrich the experience.
Pokemon Sun and Moon doesn't add very much to the game, and the new snap is not very impressive either. For die hard fans and completionists, it is still a must have, but everyone else will want to wait until the price goes down or pass and wait for the next release.