A few comments ...
1. Implicit Message-ID-s
I've personally never seen a mail message without a Message-ID, so this
doesn't look all that useful to me. But, others may have had problems.
2. A new heading field Followup-To
The concept is well-meaning, but the name is wrong. Don't reuse Followup-To
for something new. Mail is routinely gated to news and news is routinely
gated to mail, so there is a very good (100%) possibility that a mail message
with your kind of Followup-To line could end up on news and cause havoc for
millions of news readers across the globe. Some news reading software was
sent into a spin a while ago because someone created a newsgroup with a
colon in its name: such a simple thing. Followup-To is a critical line for
news software, so changing its meaning will create interoperability problems
for years to come. If you must have this functionality, choose a new name
for it.
Personally, I can't see how Reply-To can't handle redirecting replies to
a mailing list if it is really needed. Also, all mailers I've used have
two reply functions: one which goes just to From/Reply-To, and the other
which goes to everyone on the From/Reply-To/To/Cc lines. I suggest consulting
your manual to see if yours has similar functionality.
This would require changes to every mailer in existence to make it widespread
enough to be effective. Do you volunteer to do this work?
3. Use of In-Reply-To versus References
The various intricacies of In-Reply-To and References are explained in
Henry Spencer's draft update to RFC-1036. Contact him for a copy.
I suspect it is too late to add extra functionality to the reply-processing
of mail, and I'm unconvinced that your additions would help.
Cheers,
Rhys.