The current shader fallback system is pretty difficult to work with for an avatar creators. It doesn't let you to preview the end result in Unity and requires messing with the shader's internals. Most avatar creators don't create their own shaders, they use the popular ones like Poiyomi etc, so they have very little control over shader fallback behavior. Those shaders are too complicated to be graciously fallen back in an automated way, usually resulting in a completely broken look of the avatar.

(For example one of the common problems is the emission color gets copied onto the fallback, but the emission mask isn't, resulting in a fully illuminated avatar, while it should've been just a glowing tattoo)

I propose a more straightforward solution that doesn't create an unnecessary dependency on the main shader and its parameters. For any given material, an avatar creator should be able to set a Fallback Material. If set, then when fallback shaders are enabled, all references to the original material will be replaced with a references to the fallback material.

Similar to Android materials, Fallback Materials will be only allowed to use the shaders that are whitelisted as fallbacks (like Toon, etc) - it can be validated by the SDK.

This will give the avatar creators the full control over their avatar's look under the fallback shader conditions.