My first suggestion would be to work on the combat system. With Oblivion it all felt so hack-and-slashy. Let me expand just a little bit, and compare to another Bethesda game. When playing Fallout 3, I remember that the normal gunplay was okay at best, but was far behind normal gunplay in other games, but that's okay because it was made up for by the VATS system, which made it incredibly fun. Perhaps if Bethesda were to integrate something like the VATS system into TES V, maybe when you hit the VATS button, you've got a dropdown menu of various sword or axe or magic, or whatever type of weapon/combat you're using attacks, and you select the one you want to use, then perhaps even target a specific body part.
Another thing, this problem it seems plagued both Oblivion and Fallout 3, is that walking always looks strange in 3rd person, it's a small fix, but it could add I feel. Also, when I'm in 3rd person, let me swing the camera around, and even take a close view of my character, I've been running everywhere gathering the coolest armor and weapons around, and I'd like to be able to see just how cool I look.
That said, another thing that would be really unbelievably cool is customizable weapon and armor appearance. You let me customize how my character looks, but in the end he's going to be covered in weapons and armor, so let me customize that too.
Houses are a big thing, and something I felt let down with in Fallout 3 because you could only have one house. I remember playing Oblivion, I purchased every house in the game, and then decorated every one of them in a different way. There was the house that I outfitted with weapons, the house I outfitted with armor, the house I outfitted with magic staffs, potions, and alchemic ingredients, the house that I coated in jewels, I even had a house devoted entirely to beer, wine, and what litttle Skooma I could locate. This was an incredibly fun gameplay element to me, for some bizarre reason, and I would like to see it not just kept, but improved upon. Make it easier to put books on bookshelves, they were such a pain to get up there, but I can't imagine it'd be too hard to program a 'put book on bookshelf' interface. Let me move my furniture around, and give me different options for what type of furniture I'd like to have. You could even do something like Fable II, let me buy every house in the game, rent out the ones I'm not using. To make it even better, give me the ability to demolish a house, and use the land to custom build a house to my liking.
Another thing that would be really cool is a familiar. One of my favorite types of magic with Oblivion was summoning, but I remember always feeling that I wished that I could keep the creatures I summoned, and raise them. A system like this for TES V would be a lot of fun. Perhaps actually have a familiar stat, and the more you train it, the more familiars you can choose from. To top it off, make it so that the familiar also has his/her own trainable stats, encouraging me to pay attention to them. Or really get into it, some familiars would be large, assisting the player in combat, some might just raise certain stats, perhaps smaller familiars could even take up perch on the player, occassionally hopping from shoulder to shoulder, etc... Maybe some could even have unique functions, for instance a familiar for theives that could run across a room, grab a small item, and bring it back to the player on command, or one that located unique and valuable items and pointed them out to the player. But this could be done for other sorts of classes as well, an assassin perhaps could have a familiar that attacked his enemys from underground, or a familiar that he could throw and it would poison the enemy, or distract them so he could run in for the kill, or sever their achiles tendon so they couldn't get away or something. A Wizard's familiar could occassionally bring the player random alchemic ingredients, help rejuvinate mana faster, or possibly even cast its own unique spells. There could even be scout type familiars that would locate enemy positions for the player, a ghost or a small bird or something. Perhaps, getting back to the houses, let me summon creatures that aren't familiars, but that serve as pets for my various houses too.
Another thing, make sure that there are lots of unique items, and that the unique items look unique. I remember one of the things that dissappointed me the most about Fallout 3 was that the unique items looked the same as their normal counterparts. The Terrible Shotgun looked like a normal Combat Shotgun. This was something that Oblivion did a much better job with, but could do a better job still. Populate the game with lots of unique swords, axes, etc. Items with special affects that look different from every other item in the game, this makes them interesting enough that even if I don't want to use them, I'll still want to find them.
Also, make theivery more rewarding. I'm not saying that every house has to have enough loot to buy your own small island nation, but it ought to have something worth going in for. Playing as a theif in Oblivion was one of my favorite things to do, but it was so unrewarding that it got boring quickly. Most of the houses had about 300 sets of the same tableware lining the shelves, some fruits on the table, and maybe a tiny bit of gold stashed somewhere or other. Even the castles rarely had anything really worth going in for. Houses should have interesting things inside, while not every poor house should have something, some of them should have a good item or two stashed away, mid-range houses should always have at least one or two items worth finding, and castles should be stuffed with heavily guarded loot. Again, make sure that there are a fair amount of unique items to keep me interested in trying to find stuff. Another thing that would be really cool is if people frequently had interesting things to pickpocket. I pickpocketed NPC's on a regular basis, but there was rarely if ever something worth stealing apart from money. People in real life carry interesting things about with them, and people in TES should too.
Another thing, make the AI occassionally attack me too. Heavily populated areas are bound to have muggers hiding in dark back alleyways, they should pop out and attack me, and I should choose if I want to give up, try to fight, or perhaps even try to run to the nearest guard outpost for protection. They did this all the time while I was walking through the country, but certainly there would be some muggers in town as well.
