Re: So you're commissioned to write an actual Star Trek book...
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.
That sounds like an interesting premise.
"Now and then we had a hope that if we lived and were good, God would permit us to be pirates." Mark Twain, Life on the Mississippi