One thing I disliked about the Civil War quests in Skyrim was the fact that it was about 7 vs 7, with the Dragonborn. And it was pitifully fought, as the AI is somewhat terrible.
I'd like for the NPC's to work together. Some of them have bows and they stay grouped together and fire volleys at the enemy. Some have swords and shields and stay grouped up and fight as a squad. I don't like it when everyone is running rampant fighting whoever looks different than they do, it's not very immersive.
Also, the AI for Followers should be fixed, they should be able to keep up with us as we travel. And we should be able to recruit more than one Follower rather than have just 1. I like being in a party, or a fellowship, and having a team of people with me would make the game feel better. And they have AI that lets them work together and stuff, like I mentioned up above.
And the character-to-horse interaction should be better. It feels awkward to get on and off your horse, and even while the character is getting off the horse, it feels like the ocntrols are still using the horse for a few seconds. And riding a horse just feels quite awkward at times.
And I believe it is very possible. Modern games are able to support 256 multiplayer, and other games can generate vast amounts of NPC's on the screen.