The spouses of series' detectives. First of all, there aren't that many. Most of the men and nearly all of the women detectives are single. And if there is a spouse, they are usually little more than a domestic diversion. Or get killed off or divorced (Wexford). Or the gymast that was married to one of the Dutch detectives.
Can you think of any examples of spouses that are interesting, developed characters? Nora Charles, of course, to start the ball rolling.