I think that it'd be great if there was a sim like Orbiter, but supporting space combat and some sort of campaign engine.
Regarding implementing a realistic flight model in FSO, the main problems are: AI and the system used to determine object position.
The former is absolutely not suited for any flight model changes, though it can stomach some tweaks. The latter limits the play area too much for realistic speeds to be usable. Also, it causes cockpit shuddering if you go too far from 0,0,0 point, which gets annoying if you like flying with cockpits.
Unfortunately, overhauling either would be a very difficult task (though AI was edited for such model by The Minbari Project team, they released their code, so it might be worth looking into). In fact, trying to make object position system more precise might involve re-writing the entire engine.