I was hoping for this, a couple of years ago. I discovered two
obstacles.
One is the matter of legal papers. Following legal advice, I ask for
legal papers for all code that gets put into GNU Emacs (and all
FSF-copyrighted programs). I cannot get legal papers for much of the
code in XEmacs, because (1) some authors refuse, (2) some authors
worked for companies that won't sign, and (3) we don't know who all
the authors are, because former XEmacs developers did not keep
records.
The lawyers who gave me this legal advice gave two reasons: to be in a
good position to enforce the GPL in court, and to avoid accepting a
contribution from a person whose employer subsequently says it was
illegal.
As a consequence, the possibilities of real two-way cooperation on
source code are very limited. I think that's a shame, since I like
the code in XEmacs that I have seen.
The other obstacle is that *some* (not all) XEmacs developers have an
axe to grind. I'm not willing to be a doormat in the name of
cooperation, so I won't accept those individuals as coworkers.
Given this situation, what can be done?
If some of the people now working on XEmacs--the ones who can be civil
to me--want to join in development of Emacs, that would be possible.
They could eventually become the Emacs maintainers.
However, this could not start from the XEmacs source base; only those
parts for which authors can be identified, and papers obtained, can be
merged into Emacs.