I'm not sure if your v3 will be using structs to define the layout of options as this version does, but I noticed that embedded structs were not implemented.

During the reflection/parsing of the struct, if you check the reflect.Value.Anonymous (bool) field and get true you could recurse into that single embedded struct. This allows user to 'reuse' subsets of field structs defined in separate structs but embedding them in the main field struct.