If it’s not too hard to implement, it would be really helpful to have the ability to copy/paste between modules.
I’m currently designing a new wargame that mixes genres and want to pull useful code from 3-4+ different modules.
Topic has been raised on a number of occasions before now. I agree with you, but V4 may the go for that capability. (and perhaps also for map wraparound horizontally 
)
Copy and paste between modules in the V3 editor would be monumentally difficult to implement. You can already copy and paste between module buildFiles if you feel brave (or keep backups). Since the V4 file format will be human-readable, it will be trivial to copy and pasted between module files in V4.
Indeed re copy pasting between build files. I use notepad ++ with the compare add-in. Works a treat. Sometimes Vassal needs to reassign IDs, but not that many.
Well (as my kids used to say) - “Poop on a stick.” 
But honestly, that was what I figured…
Ok, so hopefully a V4 editor feature. Note I don’t want to have to go into the human readable V4 build file, but rather be able to directly cut and paste various levels from within editor. This could be a unit definition (entirely) or a single trait or whatever.