First of all, I want to thank Behtesda for including Rain occlusion, which I made a topic before SSE was released. Now I thought a bit more about things that should be in Skyrim, and in fact, they are, but were CUT from the original game. However, an update should hopefully add this,
1, Dual sheath animations with left hand weapons and staves appearing on your body. Have you never noticed how you seem to conjure up your left hand weapon when it is equipped? Well, that shouldn't be the case, since you really need to have draw animations for those things. It add immersion and dept to your combat system.
2, Dual wield blocking. Ironically, there are dual wield blocking animations in Skyrim, but they were never finished or implemented. Now they are mods that add them and they are proof of concept that dual wield blocking works, but they are not perfect. Ideally, in the 64 bit version, they should have been implemented with an universal block button, preferably LB, while allowing for LT to do regular left hand attacks. Sprint can be transferred to a different button. They are ways to make it works, especially when the Left and Right D-pad buttons are free to be used, last time I checked.
3, Fast travel system with carriages that actually move. Now I am conflicted on this, but Touring Carriages again is a proof of concept that it can work if professionally system is implemented in vanilla SSE.
Now those are things that I find SSE is lacking but technically can be made to work, especially when mods exist for them that are proof of concepts that they work. This is my opinion, but this is what I would love to see in SSE that will Complete Skyrim, or at least some of its game mechanics. Hope this is read by the dev teams, or at least pitched to them, and implemented in future updates.
Best wishes.