Picard undertakes a diplomatic first contact mission to a new interstellar republic outside the Federation. His only path of contact is through a wide spread species that is a former member of the Federation, a people who left because they found the prime directive ethically inadequate. Explores the occasional failures in maintaining the Federation in one piece.
Write this! (not to sound too demanding or anything lol) Maybe this could explain the absence of Denobulans in post ENT and nuTrek? Or however you wanted to do it