I currently have a solution with multiple projects that mostly use the same classes. As a result, it appeared to me that it would be a good idea to add a class library cont
I think this is more along the lines of what you're trying to achieve.
Put all of your common classes into a class library project and compile it to a DLL.
You can then reference that DLL in external projects. Anytime you need to add a property to it for the external project you can then inherit the class and add the property there.