The main thing that I would like to see is more structural parts. I only play with the game on stock, and I'd love it if there were some more structural panels.
I'm a bit of a perfectionist, and I dislike seeing extra paneling protruding from my builds. The square plates have been quite a lot of help in building crazy vehicles, but I'd love it if we had triangular and/or some even smaller structural plates.
It's just I'm currently trying to build a nice semi/rig and the front section is somewhat v-shaped in design, but I can't complete it the way I'd want it without having panels sticking out as it is. All I'd need is something like the small structural plate cut in half diagonally and I'd be able to complete it.