MOD based game (Minerva's Adventure)

With a comment I've gotten, I think for a MOD based system at Minerva's Adventure and other products.

I had a plan to design my games as MOD based games like Skyrim or Fallout.
However, it is hard to make all misc editable.

From above, I've planed their priority;

[1]Message Text Files
   -For translation.
   -For changing messages and scenarios.
   -For creating a new scenario.

[2]Map Files
   -For changing scenarios and maps. (Changing is more difficult to MOD-able.)
   -For creating a new map with my map editor.

[3]Image Files(Only adding or override. The original images have copyrights.)
   -For changing images.
   -For adding a new character by overriding.

Above [2] and [3] are harder than [1] to make those MOD-able.