Trying to rationalize inter-personal and bboard message headers
sounds laudable. The difficulty is finding a way to do it that
is reasonable and doesn't disrupt the installed base.
The Message-ID proposal, in effect, relies on the development of
a heuristic. This is probably not a good idea. Heuristics as
add-on processing rules are fine. As standards they are terrible.
For Followup-to/Reply-To, you are trying to distinguish "type"
of address from the header name. That is, you are trying to alter
an attribute of the message, to define an attribute of an address.
This isn't clean design. It's made worse by the possibility,
and probability that senders will need to send the same message
to individuals AND to bboards.
I don't have a counter-proposal and wish I did.
d/