It seems likely this has been requested before, but it would be very useful to be able to connect references together in EndNote, for example connect a review article to the work it reviews, or a book to another book that cites it.
Zotero has a basic version of this, but it doesn’t allow you to indicate the relationship (if you related source A to source B, source B is also related to source A). It would be more useful to be able to indicate Cites, Cited in, or generally Related. Possibly also a text field to give specific citation information (e.g. “p.85n7, re: political implications”).
A tab for each reference which lists all related references and allows you to click to navigate to those references would allow for much clearer interconnections in the database and much easier navigation.