Sneaklemming is wise, OOO is indeed a surefire cure for the annoying scaling system. Go check it out folks, it's a great mod.
As for TES:V...
-Well, keep the combat system. But work on it. A LOT. I mean, it's practically broken right now. But that doesn't mean that it doesn't have potential. Right now it's the closest we have to a real simulator of medieval sword fighting- not counting Mount & Blade's combat, although it is somewhat different in nature- and with a bit of polish it could turn out truly great. I really hope they managed to pull this off, it would add so much to the games.
Oh, and yes, mounted combat would be nice. Not a must-have (for me, anyway), but it would still be nice.
-Make the world seem more alive. I'm talking populate it with thousands of NPCs. Not just the handfuls we saw in Oblivion. The cities should be propped full with NPCs of all kinds, and the roads should be filled with caravans and groups of travelers.
Besides that, add more villages and farms. And I mean a lot more villages and farms. Would make the world less wild, and it would add to the game's sense of immersion. Increase the roadside patrols between cities, and add the odd crossroads' fort where a battalion of the Imperial Legion is stationed, to keep the region under control.
-Make monsters more rare, and deadlier in proportion to their rarity. One thing I've always disliked about both Morrowind and Oblivion was the commonness of the monsters. In Oblivion, you could hardly walk a few meters down a road without being assaulted by a troll.
What I want is for the monsters to be more rare and far more deadly in comparison. I'd like it to be so that when you run into a troll- feasting on the corps of some unfortunate traveler in a dark cave- you know that you're in for one hell of a fight, and you'll have to pull out all the tricks to win it.
Also make the monsters behave far more unique in combat. Improve their AI, make them do other things than just rush at you blindly. Trolls could pick up small rocks, scurry up on high cliffs, and try to stone you to death, while Ogres could pick up whole boulders and throw them at you (giving the player a dodge/roll skill to avoid such attacks would be nice too- maybe make it so that the player has to have a certain level of acrobatics to be able to do it?), and Imps could fly up into the sky and try to take you out with their magical powers from a distance. In short, make combat more dynamic and deadly, for both the player and the NPCs and the monsters.
-Make high skill values unlock new abilities for the player. This, I think, could be really sweet. Imagine if reaching higher levels in skills unlocked new and unique abilities- a high acrobatics skill could unlock a sort of dive/roll out of the way ability, a high athletics skill would unlock a sprint ability, higher weapon skills would unlock a whole plethora of tricks and abilities to perform with the weapon type of the skill, etc.
I don't know if this could be done, it might be a bit too much, but I think it could be great.
-And, finally, improve the magic. Make spells unique in structure, effect, and form, so that spells can take on a wide variety of appearances when cast. A fireball could come in the shape of a fiery fist slamming into your enemy, or a paralysis spell could be shown as a mist flying from the fingertips of the caster to surround the enemy, robbing him of his ability to move.
Basically, just make it so that magic is a bit more fun to both practice and look at.
Argh, sorry for the über-post folks. I admit that I got a bit carried away.
-Sorry! I forgot to add two things: Make NPC-interaction far deeper and more rewarding- I'm talking allowing the player to form friendships and romances and feuds, that actually mean something and have serious consequences, with random NPCs here- and of course make the gameworld less bland than Oblivion was. Just stick to the lore and they'll be fine.