What I immediately thought of was there being a consequence to using Undead, Daedra, Slaves, etc.
Like, what if they had a randomly generated effect where bad things would happen? Like the Undead getting a craving for flesh and zombies start spreading around the area? Or the Daedra decide to lot listen and begin wreaking havoc. Slaves could tell a guard passing by of their imprisonment and be released. Or maybe they could sabotage the place. And build unknown traps.
Part of that was covered at the end of the post, but I generally think of undead as relatively mindless puppets outside of the necromancer's influence, and unlikely to suddenly go out of control. There would be risks to those options, but I'd rather that were a simple matter of player responsibility and consequences...I'm not a fan of hand-holding. If you're careless enough to have slaves build you a monument to yourself near a patrolled area and with poor security, well, enjoy prison.
You can imagine how amazing walking into your home after a day of dungeon-plundering and seeing a huge party of bandits running around would be.
That's what you get for not providing your own security in a lawless area. If you have the resources to build a stronghold, you should have had the option to protect the place, too (or at least camouflage the entrance). Bandits would presumably be a relatively low-odds encounter, as they don't exactly have armies combing the land, and I would hope for a bit better AI than "charge blindly forward." Presumably the rest of the gang would think twice after watching the first guy get chucked into a moat full of bear traps.
But, having to spend copious amounts of money to save your home from being robbed by the average Khajiit-leveled Acrobatics would be frustrating though. Or having high-leveled thieves breaking into your home because it's very well-protected, even if you aren't famous.
High-up doors and windows would still, at least, be equal to whatever the default difficulty of the ground floor is. Just because they can get up there doesn't give them an automatically easier time getting in. The increased odds of high-level thieves doesn't mean higher odds of break-in attempts in general, just that when they DO happen, it's more likely to be a more competent thief.
My phrasing may have made it sound like thieves are a constant danger, but that's not what I had in mind. They wouldn't be crawling all over your house every night. It's based on past ideas regarding randomly generated NPC's, with a rough percentage of the population being thieves depending on the general crime rate of the area. They would each have a certain chance of whether they attempt any thievery, with where they go depending on various factors like their own ability, how desperate they are for cash, if they've heard of any easy targets, etc. Ideally, cities would also be a lot larger than they are now. If you live in a decently patrolled town, the odds of being targeted for robbery on any given night would be extremely low, and only then does it get into whether they can successfully break in. Another thing I've gone into in the past is an investigation system, so even if you were robbed, there's an additional chance of them being caught and you getting your stuff back.
The appeal of the house would have a considerable role in whether it's a valid target; if you're not famous, haven't given indications of being wealthy, keep your stuff well hidden and so on, but have high security, your house will be considered a low-profit, high-risk prospect, and as such not very appealing. High-level thieves would have better uses of their skills, they're just more likely to be the ones who come in the unlikely event that anyone comes at all. Other stuff I'd want in game would include banks, safes, vaults, and as was mentioned in the post, the ability to design secret passages/vaults into your place. As long as you're responsible, most of your stuff would be pretty safe; if you walk around town wearing 50 pounds of jewelry, telling everyone about your collections and frequently supporting local law enforcement efforts against the Thieves Guild, you deserve what you get if you haven't invested in security.
I can picture getting a visit from a fellow guild member, too. Maybe if you're known for being rich merchants with rare items could stop by. Even the possible nobility, if you're on good terms with them.
Possibly. Something I forgot to mention is that part of the idea of property is as a place for people to contact you; mail dropoff point, first place for a messenger to check, etc. I don't really like the strange current practice of messengers who simply appear behind you, no matter where you are. A home in town is easy enough to visit, but nobody's going to check the distant cave you claimed if nobody knows you're there.