Integration with the share extension needs improvement. Right now we've seen some odd bugs, and the way we modify the contents of the .hg/shared file is unfortunate. See wrappostshare() and unsharenarrowspec().
Resolve commentary on narrowrepo.wraprepo.narrowrepository.status about the filtering of status being done at an awkward layer. This came up the import to hgext, but nobody's got concrete improvement ideas as of then.
Fold most (or preferably all) of narrowrevlog.py into core.
Address commentary in narrowrevlog.excludedmanifestrevlog.add - specifically we should improve the collaboration with core so that add() never gets called on an excluded directory and we can improve the stand-in to raise a ProgrammingError.
Figure out how to correctly produce narrowmanifestrevlog and narrowfilelog instances instead of monkeypatching regular revlogs at runtime to our subclass. Even better, merge the narrowing logic directly into core.
Reason more completely about rename-filtering logic in narrowfilelog. There could be some surprises lurking there.
Formally document the narrowspec format. Unify with sparse, if at all possible. For bonus points, unify with the server-specified narrowspec format.
narrowrepo.setnarrowpats() or narrowspec.save() need to make sure they're holding the wlock.