I've been thinking vaguely about support for essentially version
controlling a quilt series, with the fundamental idea being that the
history of the branch you're working on is a sequence of states of the
series, with magic for having both the series specification and the final
state in each commit.
Note that this is a different concept from stgit (and, I think, guilt),
which uses the git engine as the magic behind the quilt-like operation,
meaning that the history of the commit at the end of the series goes back
through the patches in the series, not back through the changes to the
series.
I've got a bunch of ideas on the subject, but I don't really have the
quilt experience to know how to make this useful to people who want to do
this kind of thing. My dream, from the perspective of a user of the
results of somebody else's use of this feature, would be being able to
bisect -mm to determine first that -mm stopped working when Andrew updated
a particular tree, and then bisect within that tree (in each case
generating the test tree with the complete -mm series, but with that
tree's patch series being from the test point).