Subclass Dog into MyDog, which is exactly the same as Dog. From there, use and modify MyDog as though you would be modifying Dog. Any class that references base

Subclass Dog into MyDog, which is exactly the same as Dog. From there, use and modify MyDog as though you would be modifying Dog. Any class that references base class Dog isn't modifiable, but any future work you can use MyDog instances to get all the functionality you need (with some ugly casting). There are other solutions as well.