I think it would be cool to introduce a tree for retrofiting ship designs. I often build colony ships with no modules at all in order to save time on producing them. Then I retrofit them up to have movement modules and an outpost module. Each time I do this, I have to change the unit design for the colonizer ship. Then to make another empty colonizer ship I have to change the design back to empty again. It would be cool if I could make an empty colonizer design and then draw an arrow to the fully funcional colonizer design to indicate that I want it to retrofit in that direction. The same system could be implemented for ship designs of all types, allowing the player to quickly retrofit ships to fit different needs, rather than making identical designs for ships that didnt start out as the same design. I hope this makes sense. I explain it in action at around 20:55 in my Let's Play video:


https://youtu.be/I-lv4ci_jqQ