This would allow an easy way for people to make coloured sectors, zones, districts, different areas on the map as a transparent overlay that you could name.
If you have a large, dense factory and need to split it up from above to help you with factory management, or you just want the ability to show a friend/player, "You should build your factory this area"; I think this would be very helpful when maps gets released.
Multiplayer wise, I think this can be easy to implement. You will get a new block placement, "Map Table", "Control Center", "Map Manager"; allowing you, (as the host), to access this feature. This block would allow you choose who has permission to use this feature, as this would be the only place you can edit maps; but would update every map in the game.
I don't think you need to make the sector building too advanced. Either click and drag rectangles, or click and drag outward to make circles. If you can make more precise than that, I'm sure people would like that; but if needs to be basic to get it in the game then do whatever it takes.
Thanks for making it to the end! Hope it makes sense.