The AI for settlers is not really Hannibal-levels of refined.
I think it would be great if we were able to place down "Defense Points". You could simply go around in build-mode and drop little icons in certain places.
These would be points that settlers would run to when they're attacked. I think we all spend time developing defensible areas, but when a base is actually attacked, the settlers just run around willy-nilly looking for prime locations to have their faces blown off. It would be better if they all ran to designated points in relationship to the direction of an enemy. That way, they would hunker down behind walls, rush up to the top of a tower to snipe, run inside and fire out of windows, etc. If a settler takes damage, they rush to another point.
Hell, you could even place twenty points in a row to create a firing line. Might work well with heavily armored settlers using powerful auto-weapons.