I would like some language support to avoid having to create multiple methods for objects from external libraries that have the same methods but do not implement the same interface. For instance, braintree had a number of classes that all have the method getCustomers() returning a List<Customer>. If you could create an interface and cast these different classes to it that might solve the problem.
It might even be enough to be able to create an interface inline as part of the method signature if it only has one method that you care about. This might get ugly.
This is the use case I run into often. And its nice to have language features that help you wrap these external libraries without having to use reflection.