Normal and Specular maps generator plguins!
Using similar concepts and algorithms that i use for Laigter, i've made two plugins for godot. One for generating normal maps and anotherone for Specular maps. Boths allows to select an image inside the project, and saves the generated map in the same location than the original picture.
The specular map useful part is the shader that lets you create (very basic( specular Lighting in 2D (which is not supported by default in godot 3.x). You can use it for other textures.
Hope you find it useful!