Architecture variants for Debian / Ubuntu

Hi,

Recently the topic of exploiting newer instructions without dropping
support for older machines has come up several times inside Ubuntu
engineering. I understand this topic has come up several times in the past
for Debian as well, but nothing has really come of it to date.

I've spent a while thinking through the options and coming up with a design
and wrote some notes into a wiki page:
https://wiki.debian.org/ArchitectureVariants

In terms of building consensus around this design, I thought it makes sense
to start at the bottom of the stack and so here I am on this mailing list
:-) I guess in due course this could become a DEP, and would certainly need
to be discussed on debian-devel before getting too far.

What do you think? Have I missed any glaring implications? Is there a
better way of doing this?

Cheers,
mwh