borg backup is pretty cool, and could be a great special remote backend.
In particular it does delta compression and stuff.

There seem to be two ways it could work. Probably there are borg commands
that allow storing a given blob in it, and retrieving a given blob. And
that could be used for a traditional special remote.

But also, if a whole git-annex repository has been backed up with borg,
then git-annex could look inside such a backup, and see if
.git/annex/object/ contains an object. It could then mark it as
present in the borg special remote. This way you'd use borg to take
backups, and git-annex would then be aware of what was backed up in borg,
and could do things like count that as a copy.

--[[Joey]]

[[!tag needsthought]]