Have you ever thought warhammer 40,000 terrain should be more like lego sets?
No? Me either, until I bought this set and then I realized this is the solution that 3D printers should all be using for these big buildings.
Corvus has put together a great set of modular walls and corner posts which can be constructed in basically endless ways to build the kind of science fiction city that you envision. There’s heavy industrial and very gothic style leaning designs in the set to pick from so it’s easy to build into the warhammer universe.

Lets talk selection of buildings. Trust me you wont get bored having the same details repeated over and over again with a board built out of this set. There’s 60 different walls to choose from with different decorations on each of them.
My favorite kinds are the ones which have a different inside and outside wall, because these can be printed and then pick which side you want facing out later. It helps to give lots of variety to your printed collection without having to print every single wall multiple times.

These may not be free like the stuff on thingiverse, but the price is reasonable and there’s enough variety in the walls that you really wont need anything else in your collection although you might want to keep buying more just because the corvus stuff works so well together.
A couple of the things shown in the pictures above may be from his other sets or thingiverse if you can’t find it in the bundle.