Some kind of big case with a moral dilemma that ends with something big for McCoy (who, although only one of many on the show, is the main attraction for many people). Either he retires, gets promoted, or wins something, I don't know. Maybe something good for Van Buren as well (promotion, maybe?). The cast regulars celebrate, but the cops get called out to investigate the case and the ADAs get ready to join in, showing that life goes on in New York City. Then they pan out to show the entire city and it fades to black.
Part of the advantage of this is they could revive it any time they want and it doesn't seem unrealistic. But, if they give closure to the characters who have been on the show the longest, it'll feel like something a bit more substantive.