The first suggestion about Structure looks somewhat normal to me, though making type hints valuable at runtime seems a little bit non-uniform with all other Python type hints which are not meaningful at runtime. Also there is a need to maintain both syntaxes and make them mutually exclusive, but it’s up to you.
But the second suggestion with decorator is very confusing. Please imagine if you are a newcomer and trying to understand why there is an empty function, maybe I need to remove it. 
Static analysis may decide something similar. And what if I add some implementation instead of pass statement? Will it be ignored?
Another argument against the decorator is that a dynamic library is pretty low level object, while a decorator is something at high level: it’s typically a transformation of decorated function, but you suggest something different and strange. I think it’s not worth it.