I don't like Seven. I respect it as a good movie, but I don't LIKE it, but it had a perfectly appropriate ending and you're correct, no sequel should ever be penned, produced, or released.
Mills is found innocent at his trial for the murder of John Doe by reason of temporary insanity. He can no longer pursue law enforcement as a career and has become a solitary drunk (cliche #1). Somerset, who is in a new relationship, drops by to see Mills from time to time but the visits are more painful than comforting.
Meanwhile Somerset begins to investigate a strange new series of (gasp) serial killings (cliche #2) in which criminals are dispatched in gruesome fashion, and clues at the scene lead to the theme of "wrath". Duh Duh Duh! Somerset suspects Mills may have gone over the edge (cliche #3).
Seven 2: The Piece of #@$%, coming soon to a theater near you...