a small type-field for 'Notes' relating to what a specific layer is doing. Often when sharing .spp files across team members, it is difficult to ascertain what each layer is responsible for.
Think of Maya's Notes section at the base of the Geometry's Attribute Editor.
As this would be a 'per-layer' feature, a small icon on the layer could indicate that there are notes attached
I can see how this would be a great idea... but I would be happy per Substance Painter File or per Folder basis. I think it would be overkill to do per individual layer... Just my thought. I sometimes keep a LOG of what work needs to be done or what I have done for the file and I can see how that would help.