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.
"I'm creating a (free) universe, just a hobby, won't be big and professional..."