Having been through this sort of headache a couple of times, I'm convinced that the coupling of functionality must be as loose as possible. It sounds like OP's

Having been through this sort of headache a couple of times, I'm convinced that the coupling of functionality must be as loose as possible. It sounds like OP's solution follows that approach, although he's a bit thin on specifics. Another useful concept in this context that is usually unknown (and often partially reinvented) by C++ programmers is protoypal inheritance. C++ makes this anything but simple, and you essentially have to reinvent your object system. It can be done, but it's not pleasant. I'll need to try these things in more expressive languages sometime.