Like
@Kohlar the Unkilled said, variation. And on the note of animal variants - what about more animals in the wild in general? Not
everywhere to the point that you can't walk a trail with a sheathed sword until you clear an area (oh, and definitely have animals re-spawn more), but definitely more than was in Skyrim. More like Morrowind, I guess. The number of creatures/enemies that outclassed me was high - and I loved it. But, conversely, there were plenty of low-level creatures I could deal with that let me figure out how to play the game, and gain experience points to boot. And with the increase in creatures/animals/random encounters, there should be more fighting amongst the NPCs. Wolves chasing deer, bears duking it out, etc., etc..
Oh, and bandits should act like bandits. Roam around, get drunk (yet another thing that should be added - alcoholic inebriation), attack civilians they find on the road, and most importantly - occasionally fight each other. There should be arguments sometimes, and rarely, ones that cause them to get into a fist fight (or even a fight to the death). It could serve as an occasional distraction, and the PC could slip into a bandit fortress while everyone is cheering/goading the fighters.
Also, not sure if it's been mentioned, but add named NPCs. Even if they have to recycle names, they could come up with thousands of names that we see. It would let the bandits trick us more often, and it would even make it a bit harder for some people to slaughter a whole fortress of bandits when they see that all of these characters have their own name, and have human interactions (Again! Add this too! There should be more and more realistic interaction among NPCs!) with each other. Plus, that would open up more interactions for the player to make. From realistic conversation (not just "Give me quest. I done. Pay me. Bye.") to arguments, which could even develop grudges (take something from Shadow of Mordor, I guess? That would be cool.), and even sexual intercourse. Obviously there'd be no graphic scenes, but they did it in FO:NV. It could be used to further develop relationships (or destroy them, if you had a spouse and you cheated on them). Hell, you could turn it into a business - pimping your own character or NPCs you had control over.
Final thing - like I said, add business. Make banks, let PCs open shops and run them (there could be a menu that enables you to decide how realistic you want it to be. From budgeting and cleaning and purchasing goods yourself, to an autonomous machine that just makes you money), make it so you can create an empire of your own. And, of course, make enemies of your rival companies. Like Fable's system, only way better. Ya know?
Anyways, that's all I have for today.