Basically as the title says. I know there's plenty of threads on the matter of specific homestead aspects but I wanted to bring up a few (probably well-trodded) ideas of my own. (and I didn't notice any threads for suggesting new player houses in general)
So, my list of ideas to add to the already bloated suggestion box goes:
1. House Ships: pretty self explanatory, but to give a little more detail, I'd love to set my characters up on a full-sized ship like The Prowler, The Lydia, or the Spearhead. Does it 'need' the function of being able to go to different ports? Honestly, I don't think so. Some might disagree but I really don't find it necessary, I'd more just want the ship to be out in open waters.
2. Hybrid Styles: This is a more complex one but would offer Zenimax plenty of new ideas. Basically, Tamriel is a place that's had lots of different cultures, and all of them have fought or invaded one another at some point or another. When that happens, it's not always effective to tear down the enemy's old buildings and put up new ones, but it's not too hard to add to it later on. So basically, where two cultures of tamriel ended up overlapping and occupying the same space, sometimes that space got an odd blend of different architectures. To give a list of ideas:
a. Bosmeri-Ayleid home (Valenwood home pods growing out of/filling in the broken pieces of an old ayleid ruin)
b. Redguard-Dwarven home (Sometimes a Redguard noble stumbles across the ruins of a dwemer city exposed above the sands, filled with deathtraps and incomprehensible machines and thinks: "Enclose this space as a nice Rihad so I can't run from the death machines and this will make an awesome house.")
c. Argonian-Dunmeri home (In the wake of the War of Three Banners, some of the find homes of the Great Houses have been wrecked. And the time and money it would take to restore them is too impractical and thus the Dunmer nobles end up selling to their former slaves to make some coinage return. And since the Argonians are more pragmatic and their styles of architecture are quicker, they're happy to just add on some literal mudrooms and call it a day.)
And so on. Really, even if there's never a major conflict between two races/cultures, there's still room for individuals to stomp on one another, take the other's house, and add their own personal twists to the architecture and design.