OpenNTF Mail Template - Feature Request: Team productivity email
It sounds like a good idea, until you implement it, and realize you've taken a standardized interface like email and turned it into a proprietary haven for functions.
If there's a plug-in from the other direction, that's a different matter. But anything that adds non-email features to an email form is asking for long-term demand for an endlessly tweaked interface.
Honestly, if this kind of collaborative workflow is needed, then it should be developed as a collaborative workflow application. If you need email notification, that's what doclinks are for.