Kevin,The problem that you run into in this arena is that plug-ins vary so much from app to app. In our medical system, we have implemented a plug in, so to speak, that allows users to download and purchase themes and then implement them. Granted this is a simple type of solution.
The only way you can really make a plug-in work is to create a set of classes and interfaces that allows other applications to be developed towards you application. If you create the foundational classes, it would be relatively easy to implement at that point using reflection and the activator classes.
Sorry this is a vague response, but there really isn't a defined standard for this type of implementation. 