Please make --backup-dir option to store differences in rdiff-backup' style repository. I.e. not complete copy of backup file, but binary diff (like xdelta3) of backup version up to the new version. That way backup dirs will be much smaller if actiual file change of big files is small.

Of course there is still problem of restoring, like in plain --backup-dir, but that is different question. And that new feature will be good step in advance of better backups with rsync.