I've read somewhere that in SC2, some experimental functionality comes online before the thing is completely finished building
So it would be interesting to have, let's say, a spiderbot that can start walking before the cannon is ready. So you could keep it in the base while you assist the laser upgrade. Or you could send it to the front and the spider would finish building the cannon on the way. That would get it out the door sooner but it wouldn't help you to defend a base sooner. So it would essentially reduce build time in some situations but not others.
Or a different approach, when the GC finishes building it has a movement speed of 0 but a fully-functional cannon. Then you assist it with the "movement speed upgrade." When that finishes, it can start walking. So you could use it essentially as point defense until you finish making it. I see all the time people scrambling to finish a GC while enemy units approach. It would be interesting if the GC could shoot at them while it was finishing.
Since these upgrades would be only a net positive, they would start building automatically when the unit finished construction.
hey could be paused if you don't want to spend on them but they couldn't be canceled.