For me I would create a massive mansion. Including a big underground basement. My basement would be full of the richest wine in the land and a bar serving drinks and food. On the first floor I would have a big long table full of food and drink with all the most expensive tankers and plates etc and thrones for me and my wife with a big dragon head mounted above and behind the thrones. The room would be full of stuffed animals from all over the land. I would have all different themed rooms full of settings from like the main room of the dark brotherhood, a setting from blackreach, a drauger room etc you get the idea. I would also have a dungeon full of people chained up and locked in cages. For the second floor I would have everything an adventurer would need from a weapon room full of every single weapon in the game to a mages room full of every single ingredient to mix potions etc. Then the third floor would be loads of bedrooms including the master bedroom for me. And all around the house would be decorated with skyrim flags of all sorts. I would also have indoor waterfalls going down the sides of the walls. Also outside I would have a training ground for archers etc. I would also have everything you would need to create weapons and armour. Thats all I can be bothered to type right now. Ide also have a big farm and a horse stable full of horses and all the domesticated animals of skyrim.
That I could say Hi, Bye, Thank you, No thanks, You're welcome, etc. to the people I meet and/or talk to. Sometimes I find my chars so f**king impolite.
Being able to make my own questions and answers would be nice too, but needs a very sofisticated AI I'm afraid.
Making my chars actually speaking their lines with the proper body language would be awesome.
I think the hi.bye thing should be there and wouldn't be that difficult to implement but creating your own questions and response would be a nightmare to program. The size of the quest system even with the Radiant system would just explode and I would imagine you could end up wrecking a particular save there is that many criss cossing quests etc. A nice idea but I dont think it could be viable in Skyrim.
Hmm, lets see.
Dragon riding with functional aerial combat
Mounted Combat (to combine with dragon riding!)
Bigger Civil War sieges
More dynamic civil war (mount&blade style)
Pretty much all the game jam stuff
and finally something I might actually be able to do, add new 'combat stances' which give you a combat style that grants some bonuses with a few penalties (like damage reduction with decreased damage output, attack and movement speed with decreased protection)
I've said it before, all races should be marriable and there should racial children and allowed to have children or adoption. I just think the child part would help with the whole roleplaying exerience, think about coming home to a true family when you come home from an adventure. The reason i say all marriable races is because I play as Khajiit and I think a Khajiit should marry a Khajiit.
As a reward for your civil war you can be made Jarl of one the holds to replace someone who was not loyal to your respective side. A whole new quest line involving being a leader with political intrigue, Game of Thrones style.
The rewards for either side are underwhelming. It might do them good to keep the dragonborn happy.