It would be nice if decorators could be passed an AspectInterceptorBuilder and could choose to advise methods. Such a decorator method could return void, and would not need to have the delegate passed in.
Difficulty: what is the order when a single service has advised methods and interceptors (via traditional 5.0-style decorators)? All the advised method changes would be grouped together and the outcome may not be as desired. It may be required that any single service have 5.0 style decorators or 5.1 style aspects, but not both.