Sorry but with all the higher value things to work on this is not something likely to become a priority any time soon.
Deleting pages from a web site should in general be an infrequent at least for public facing pages and I'd rather err on the side of not losing content so that things "deleted" by mistake can be recovered.
Implementing what you are asking for would be significant work. One thing that we may do in the near future is make it easier to find the content instance that are not published on any pages.