I want to develop an in-game editor

My game is a traffic simulator.
My original idea is write an ue4 plug-in and provide it to players then they can create their own map.
But i found ue4 editor’s size is so large.
So, I want to develop an in-game editor to build a map for players.
the editor can place any objects( building, road, street, etc) to build a map and save it to hard disk.
And the map file can share to others.

So, how can i do to edit and save the map in runtime?