Some platforms, slope blocks, corner blocks, rounded blocks, griders (I know they are troublesome for graphics performance, but they look beautiful), 6-sides connectors, etc.
They would make creating bases, space stations and complex craft much easier. I know some of this can be achieved using hacks with empty fuel tanks, but having a proper way to do those things would be nice and would open up much more possibilities (especially if each of those parts could be configured).