flight model... hm about the only thing another engine could add is lateral thrust with visuals, sure it would be nice if barrel rolling worked right, but i don't see what else you're asking for except perhaps better AI, or ship/equipment tradeoffs similar to Allegiance's mass/stealth properties on equipment.
i wish you luck in your endeavor to build a new game, but I hope you'll open your mind a little to critical opinion. Sometimes realism is sacrificed for gameplay, but its ALWAYS compromised for playability. Want realism? Play total war or whatever that ioncross guys mod is called; very real and very not fun -> well, unless you like losing all the stuff you worked for when you die(now that's realism!). There is a median that is fun, and FL is IMO right in that spectrum-> systems are smaller than true scale to keep player interaction high and short-attention-spans focused on the game, ships are tight handling(referencing inertia, not rotation) to allow a more 'shmup-y playing style, travel times are long to give the universe a feeling of size but one can still travel around the galaxy several times in one MP session. Everyone thinks they can do better, but it's up to them to put their money and time where their mouth is and prove their claims to the hordes of gamers who already have their own very specific ideas of whats funnest in a spacesim.