Inner Line Art

Before diving into this method keep in mind that while you're modeling you have to think ahead and place edges where the line art should appear! 

So, you have finished modeling and you have to start texturing. First things first we create 2 UV sets. One for the "Texture" (Base color, Shadow, etc.) and one for the "Line Art".

You can unwrap the Geo normally for the "Texture" while we will have to think in "blocks" while unwrapping the Geo for this Inner Line Art Map.

The Geo has been cut into blocks. Each cut will have Line Art. 
UVs MUST BE SQUARISHED OUT IN ORDER TO MAKE THIS WORK!

Inner line art is driven by a B&W Texture. The texture can be created with a low resolution (512x512 is good enough).
It will be created  with horizontal and vertical lines, NO DIAGONAL LINES!!!
We do this in order to follow pixels' direction which won't cause a quality loss.

Once we have imported the texture, we start moving the Sets over the black line. 

Then we start moving the vertices that touch the line in order to create thickness variation or to make the line disappear.

If you close the gap between the vertices that are on the lines and the one next to them the line will appear thicker.

Let's say you have to add another line inside this block, but you haven't cut that edge. How do we resolve this without having to unwrap this block again? 
With Face select mode turned on we select the section that needs to be separated, we press (V) and then we move the section.
As you can see the section is now separated from the block it was attached to. If you notice you won't get a red line (marking UV cuts) on the geo.

We keep repeating the same process until we have the whole Geo done! 

The method is pretty simple and straightforward. If you have any issue hit me up and I'll help you out! 
Keep modeling! 

Peace.