I thought about returning a special subclass.
Future has too rich API: get_loop(), done(), result() etc.

What's about returning the proxy object with future instance embedded; the object raises DeprecationWarning for everythin except __repr__, __del__ and __await__, __getattr__ redirects to getattr(self._fut, name) for all other attributes access. 

It is a more complex solution but definitely 100% backward compatible; plus the solution we can prepare people for removing the deprecated code eventually.