There is a list of renderer features that are yet to be implemented but not strictly necessary for the engine to function.

• OpenGL
  • Use 4.x features when available (optional)
  • Support ancient 2.x legacy (optional)
• Vulkan implementation (Support Vulkan graphics API #242)
• Render pipeline abstraction
  • Add support for OpenGL uniform buffers #1053
  • Circle geometry
  • Mesh importing
  • Renderbuffers
• Functionality
  • Screenshot support with PBO optimization for texture downloading (optional)
  • Outline rendering in shader
• Terrain rendering
  • Cache blending results (Cache terrain blending results #154, Cache blending #158)
  • Do as much as possible in shaders (Implement terrain blending in the shader #149)
• Render pipeline optimization #1506
  • Occlusion queries

(Taken from #34 (comment))