RAML (see raml.org) is pretty sweet as a design tool... like Swagger 2.0, you can design your full API and corresponding documentation in the designer tools.

It would be nice to have ServiceStack support reading RAML generated specs and automagically create the corresponding ServiceStack API with stubbed in methods.

One of the nice things about RAML is the ability to generate a mock JSON responses and/or status codes from a specified endpoint as directed by the RAML designer tools. This allows it easy for other peer developers (and regression tools) to consume your service although it might still be under development or is incomplete.