I'd have zero problems dropping 500,000g or more to fully upgrade Castle Volkihar.The docks on the western side,the undercroft,the north and east towers and the ruins all scream 'renovator's dream' to me.
Yup, big real estate should absolutely be added to Skyrim. The option to spend hundreds of thousands of gold will be a great gold-sink for the game's economy. And the ability to build or improve housing will be something I think many players want.
Agreed. I would LOVE to renovate all the closed off wings of the castle, open them up for usage again. I'd also like to be able to bring more thralls and "freshly-turned" into the castle as followers and agents. One thing I really liked about the Vile Lair DLC for Oblivion was the minion you could order to "go out and murder in the name of Sithis". Something along those lines would be
fantastic!
I would also like there to be a way that I can either steal the deeds to houses, farms, etc., or petition for ownership of locations to the respective Jarls. I mean, if a certain individual "happens" to expire the leave that place empty then why can't I pick it up? Maybe keep it or rent it out, or resell it for a profit? lol. It would be fun after all. Even if the current owner were still alive, it would make an interesting side-quest or mini-game. Buying out the current owner, persuading them to sell through "persuade" or "intimidate" options. Think of the possibilities! Imagine eventually owning an entire city. The Jarl paying you monthly tributes. You'd be a
Don! lol

Just a wild but fun idea
Also there are some neat and interesting places out in the wilds that I've run across that I thought would make excellent "homes" or "safe houses". Like a small hatch behind a stump on a tiny island out in the middle of a small lake I recently found. It leads down into a small smuggler's den, which I ended up clearing out. Once cleared it would be cool if I could file for a deed of ownership with the Jarl of that hold for that lake area, then purchase some upgrades when it clears! But wait too long and bandits move back in.