Here's the GP branch for 2.8
Docs:
- https://wiki.blender.org/index.php/User:Antoniov/Grease_Pencil_as_Object <--- High level design docs (some may be slightly outdated)
- https://developer.blender.org/T54426 <-- Overview of the main changes made, and how they relate to each other
Changes since last review (in ~October 2017):
- GP Object's data is stored in ob.data not ob.grease_pencil (ob->gpd) now
- Modifier stack handling/usage has been refactored
- Lots of code cleanup & review
- New features
WIP stuff:
- Restoration of simple GP "Annotation" functionality for 2D editors