To step it up a bit, the no build limits has me thinking of planet building. Now I know that the way that the Magrathean's did it was pulling matter though white holes, and I have seen nothing so far on spacial anomalies, (and for this to be a thing you will probably need the ability to create them) so this is way out there. But limitless should mean limitless, although it depends on how the core and internals of the planet are going to be handled, and the way the movie depicts it at least it is a bit ridiculous (I don't remember how it was described in the book). But the logistics and raw scale of the ships involved is fun to think about. Of course it doesn't have to be an industry, it could like a throne of sorts for a mighty ruler of an alliance of organizations. Just a very out in the almost complete emptiness of space idea.
I should also point out, Magrathean's are from the book Hitchhikers Guide to the Galaxy, and they build planets.