The most important thing I'd build into it would be height and the largest doors that would fit in the wall. 20 feet of unrestricted height would be best, but that's probably dreaming.
The next most important thing I'd build in would be a gantry crane heavy enough to pick up and move the hull and keel of, say, a Dragon. 1-5 tons is probably enough.
After that would be enough light to require sunglasses at midnight, then three phase power and finally six inch dust collection poured into the floor.
After that, I'd be ready to build anything.