Define new topic types in the system, with their own configurable xml templates. The new topic types should be subclasses of the existing base topic types Concept, Task and Reference. So that when a processing system encounters a topic type for which it has no information, it will treat it as the base class.
A couple of additional factors: 1) Sub-classing new topic types from the three base types is important for forward-compatibility with DITA. 2) The Content Spec Processor will probably need to pull a list of supported topic-types by name from the server to validate a spec. I'd be happy to get extensible topic types with templates before CSP support for them is available, because I can create a book using the base types, and then manually set the subclassed topic types in the Skynet interface.