Since you're looking for ideas, I think more flexible building meshes would be fantastic!
For example, if you ever load up the solitude meshes in nifscope the backs are usually missing!
I'm aware they did this because we never really see them, but it does kind of make things a little harder for us modders, and I think a resource of 'full' buildings would be highly appreciated!
Oh this, this, this, this, this, this!
I hate it when they custom tailor things for the vanilla game only and limit modders even though finishing those meshes wouldn't have been very difficult for them.
And personally I'd also love to see some nicer upper class clothing like the Shivering Isles dresses but I know this may be a tall order ^^