In addition to other changes to how parts are attached, I propose being able to turn off individual attachment points on a part. This would make construction of sub-assemblies easier.
A similar suggestion has already been made, which is to have the ability to click on the 2 attachment points of each corresponding parts you want to be joined. I don't think both my suggestion and that one need to be implemented, I think either one or the other would suffice (probably whichever is easier for the devs to code in or implement). Here is the alternate suggestion: https://www.simplerockets.com/Feedback/View/U73McP/Attaching-two-parts-by-manual-selecting-two-attach-points-on-them