Unfortunately X-Wing's engine cannot be used for fan projects because you can't mod it that easy. (IIRC)
It's also very, very dated, without the benefit of the many upgrades made to both the Vision and Freespace engines over the years. I'm not sure what advantage could be gained from this, either - it might be simpler to just make a clone.
Regarding the Freespaces: One of the main things that sets them apart from the Wing Commanders and Starlancer knock-offs - and doesn't seem to be noted as much - is the amount that they rely on scripting to move the game along. It's been years since I played either game, but the memories that endure are of the "good job fending off those twelve bomber attacksohnoherecomesathirteenthwaveandsixcruisersclosinginallaround!" or "dive right through the super-battleship flak to take out the turrets so that our capital ships can jump in and hit their engines before another battleship jumps in and disables them so that you have to run a gauntlet of enemies to jump out!" (The analogy that comes to mind when typing this is some old arcade game like Robotron 2084, which I might make in more detail if I had actually ever played the real Robotron 2084 rather than freeware knock-offs.) It seems like there was a conscious decision on the part of the designers to reduce the 'formula,' such as it were, to its bare essentials and turn the game into a frenetic gauntlet of enemies. This did, I have to admit, make for some entertaining missions, but the inevitable result is that it makes the game absolutely linear. Every Wing Commander had a strong degree of interactivity in the way you progressed - even IV, with its rigidly overpowering storyline, had the defection paths and alternate missions sets. Freespace 2 only had one "accept-or-decline" mission branch, and in both games every mission is win-or-die; you don't even get the option to eject (though perhaps tellingly 2 did give you the option to skip missions after the fourth or fifth try.) It gave the game the replay value of an adventure game at a time when adventure games were on the way out, and I believe this is what prompted the designers to release both games with mission editors, hoping that the user-created content would make up for it. It did eventually, of course - five years down the line, far too late to save the franchise.
As an aside, it's funny to see Renegade mentioned in this thread, since it was one of the first games I picked up after Wing Commander and for some reason a lot of my memories of it are mixed with the Freespace demo in '98. That's a sad story, there - it had one of the most elaborate tabletop game backstories behind it (I still treasure the manual) mixed into an awkward and uninspiring space sim. I recall there was a sequel intended, but it fell through, and so did the Renegade franchise itself, dying even before FASA did. Really a shame, because it's one of the big, bold, archetypical space-opera stories and so much more could have been done with it. (It does have one direct connection with Wing Commander - David Ladyman worked on the pen-and-paper RPG.)
Ultimately, though, the one other space combat game I have any real attachment to is Independence War, mainly for how incredibly different it is from what we usually think of as "space-sims." You don't fly a fighter, you fly a corvette, and you handle it like you would a small capital ship - managing systems, tracking and firing at targets in three-sixty degrees, and Newtonian physics (which I imagine is what scared off most people.) The setting and story border on the dark, depressive part of science-fiction which I personally detest, and yet the enemies you fight are flamboyant space rogues that paint their ships yellow with graffiti. The whole thing feels strangely realistic, for what little that's worth; looking back at it, it almost feels like Battle-Capsule Apollo. And yet, there's all kinds of bizarre decisions that drag it down; after watching the ten-minute introduction you load up the game to discover that there's no music whatsoever. How does that happen? It's also not the easiest game to run on modern systems; the last time I played it was several years ago, when I had a computer primitive enough to reduce the memory cache to 400 megabytes (in retrospect, this may have been for the Glide wrapper.)
What I'd like to see in a future space game is more interactivity and more dynamicism in the way missions progress)what people sometimes call the "Metagame" and which I'll use here too because no alternatives spring to mind) and the prototype is right
here. I often go back to this, thinking of how much more dynamic things could be today, instead of having to rely on scripting and mission branching. Starshatter does a little of this, but my (admittedly brief and demo-only) impression is that it flies much more like a 'serious' flight sim and isn't as immediete and engaging as a Wing Commander. Still, the idea of a truly free-roam space-sim, with serious financial backing to give it shine (and the occasional "plot mission" digression to keep it from becoming entirely too abstract) is an engaging one. The actual ship-to-ship combat system wouldn't change too much; what would change, and what the emphasis would shift to, would be the framework surrounding it, giving it an element of strategy and freedom that's usually attributed to Privateer games. In effect, instead of Wing Commander: Armada, Wing Commander: Wing Commander.