Make your own guidelines to anyone who wants to mod your game, I would suggest you:
-Editing original sprite sheet. (No Load image by URL due to memory problem, you can clone 5 objects for future new units).
-Create new object using instance variables to setup their attributes, use XML.
-Map editor, save to Array and load the selected .json file.
-Use XML or JSON to read selected audio file name, everyone can edit the game using XML or JSON files without Construct 2 but you have to define the public variables to adjust your game.