I wish they would make the carriage system more realistic, have it travel between towns because quite frankly nobody is using it anyway cause your paying for fast travel... they also need to add the missing carriages to all the minor holds, it doesn't make sense that it takes you there but you can't go back.
.....This could add things like: having arrows weigh you down, needing to sleep/drink/eat or DIE, camping outside (for easy access to beds) and others.
If a game has mods... don't buy the console version

For both the carriage and the camping/eating things, both are mods available for PC. The carriage one is... lacking. I think Bethesda needs to take over and help the mod author out and implement it in the next expansion. The link between the horse and carriage is kinda buggy at the moment. The other mods, there's IMP's more complex needs, which requires you to eat, and there's three different camping mods I know of. There's also one that makes weather effect you (you'll freeze to death up by winterhold in a blizzard if you don't make a camp and sit by the fire... or go inside a building/cave). I run with all three of those types of mods. The hypothermia one I use turns off fast travel (togglable), and having to eat, sleep, travel by horse/carriage and stay warm adds soooo much to the game.
I'm not one for the big changes, but little things I've seen in other mods would be great. For example, why, by default, does the player's spouse just sit at home and lounge around all day (if they aren't a usual follower)? Ysolda is LAZY. There's a mod that gives your spouse a job in Whiterun Market. Amazing even though that's the only town it works in, but it should be in the game by default. If you keep your spouse in the town they are usually in, they should stick to the same routine they did before you married them, aside from going home earlier if necessary. For other towns, give them a storefront to that little shop they brag about. A little immersion tweak like that would be great. Big game changers I'm not so good with the ideas on.
Speaking of Whiterun, Breezehome by default is terrible. If I was the Dovahkiin, I would have ripped that house to the ground and built something much better on top of it. If a new home is added, please make it worthy of a dragon slaying machine.... but since "time has passed," maybe make the default home a little bigger. Hell, make it a new upgrade and worth a good sum of gold.
Those immersion mods above should be added, toggleable though so those that don't want to use it don't have to. Adding something for camping would be great, because setting up a camp can be a chore, you have to find decently flat ground, which is actually kind of hard. Fire pits mold the ground around it and flatten it out, but tents don't, which can make getting in and out of the big ones a bit of a pain if it's on a slope. In my opinion, have tents flatten the ground under it, even if it "digs in" a bit.
One last thing: either give an option to toggle their voices off completely, or give followers way more to say, and more personality. They are annoying at this point. And when I am crafting something in the house or just standing there for whatever reason and they are close by, they shouldn't say "I'm right here/I'm right behind you/I got your back" (Lydia) every 10 seconds. Once every 5 minutes is fine.