The above is just a mockup created in GIMP of a hypothetical building. Did some looking, and hand-digital-painting buildings wouldn’t be worth the effort. However, I think tiles might just work fine for making unique buildings. In making the above I sort of went about it as if I were trying to make it algorithmically–mapping out hallway/room intersections, then floodfilling in some tiles to see what the result would look like, and well, it may not be “City built out of ancient husks of crab people,” the final result definitely doesn’t look like a bland old rectangle, that’s for sure.
Back to Morrowind/Oblivion, from my time playing with its built-in Construction Set I do know that their levels and such are all designed in terms of building blocks: t-junctions, 4-way junctions, corners, short hallways, long hallways, etc. Perhaps that idea could be utilized, in that not only are tiles the building blocks of a structure, so are “prefabs” defined in some external file as arrays. Then building styles could be applied at map generation that have weighted choices, so that style A has a higher chance of generating winding roads with clustered buildings intermittently, while style B has one or two monstrous interconnected structures that sprawls across the entire 70×70 grid, for example.