I have done that for one game. If you want I can send you the code.
In my game, I made level design with json file.
All object have a uid and when I want to display it with my json, I make an object with the uid, the x, the y, the scale, the rotate. Everything it s define like this.
In my game I have unit who spawn in a special order. In an array, I write the uid of the unit, the delay and where it spawn.
I hope you understand how to make everything display with a json.
How the user generate the json?
On map editor, the user have only visual button. He select the unit to display with a button and then click where he want the unit to spawn. I record the uid of the unit, then the x and the y and when he spawn the unit.
For map object, the user can't do it but it s the same thing.
At the end of the map editor. The user can choose to save the game and have to write a string for the savegame name. Then the file is send to a server.
When a user want to play this level, he have to write the level name, the game will search on the server the json file and load it.
Map editor for player are very simple but it can be really help full for you to make a very powerfull one for you. Like this you will be able to create new level in a few minutes. With the keyboard you can write the id of the object. Click on the screen to set the x, y... I have done that to my game and it really help me for the level design.