I'm not level 50, so I don't know if things change as you get to a higher level, but so far my experience with space combat is that being stuck to what many forum posters are calling "rails" is a bit of a downer. It would be nice to feel like I'm actually piloting the ship, not just playing an arcade shooter.
That said, I think it is unrealistic to expect a complete revamping of the system to have "open space". That would take a total redesign, and likely be costly. So instead, I would like to make a much more modest suggestion that could probably be implemented more easily.
Have multiple paths (or rails) in the same mission. Make it so that there's two or three possible directions your ship might turn at a certain junction, and you don't know which one you will take. A more advanced version of this idea might make the choice of path somewhat dependent not just on chance but also on the user's actions (are you strafing left a lot? then you take the left side around that asteroid....). The spacecraft would still be on rails, so you don't need a complete total redesign, but the randomness/choice will add a nice new flair.
Of course if you can go all the way and make it open space, hey... I won't complain!