When one downvotes a question, he usually does so because it is low quality, but still considerable for the site (when it's not considerable, downvote should be avoided and question closed). It would be nice to have an option to get notified when such a question is edited; it likely means that it was improved and the downvoter might consider to revert his downvote.
The design could be something like this:
N symbol would "lighten up in orange" like the arrows or the star do. (I'm not a designer and a letter is probably not a good option, but graphical design is not the matter now I hope.) The notification would be switched off automatically when the downvote is reverted (and the
N button would disappear), and could be switched off manually of course.
The availability of such option just next to the downvote arrow would encourage voters to return to the posts they downvoted and reconsider their vote, and in the end, it would encourage post owners to improve their posts (because the chance of the downvotes being reversed would increase).
This might be used for answers as well, but they are surely not the primary concern.