If the "roadmap" is to be simply a list of the MIME-related RFCs, it
would be of little value except perhaps as a working document leading
to consolidation.
if the intended audience is new or recent implementors, a list of RFCs along
with recommendations as to which of those RFCs need to be implemented might
indeed be useful.
such a document might also provide an opportunity to resolve conflicts
between the different specs, emphasize things in existing specs that need
emphasis, provide implementation advice, etc.