Hi! I would echo @micuat 's feedback. Shadow mapping is complicated to implement and somewhat counter to the purpose of the project. Our goal is not to build a fully-featured WebGL library (which threejs does really well) but to create a tool for 3D experimentation for new coders and people who like the Processing/p5 API/philosophy. So this movement towards realism is likely more work than is justified by the value-added to the library’s mission. I would drop that section.
The multiple materials proposal is a big one and I think it is a great centerpiece for your proposal. It would be a good idea to reflect on how bringing the expected behavior with materials closer to P3D might improve overall accessibility. This can be as simple as stating directly that features that don’t work as expected can cause frustration and that frustration may be mistakenly placed on the learning process instead of the library for new coders.
I would also look into how Processing is handling this problem and include results from your research in your proposal.
Hope this helps!