Well, now that you've said that, Bobb, I feel a little more comfortable about making this post.

What is an FPS. It's basically a big fat model that other smaller models move around on top of. We can make big models. We can make small models. We can make a skybox, and we can make small models that move around.
Next you have the classic gun and HUD that's been around since Doom. Yep, we can do that with the ingame cockpit code, make a model and set the eyepoint correctly.
Now you have to make the stuff walk around, jump, shoot, die, etc. This is the part that Freespace can't do (along with gravity). If something gets hit, it has to disentegrate in a ball of fire.

For walking, you need some sort of looping skeletal animation with start/stop segments.
For jumping, same thing.
For shooting, you need an animation that plays when the gun shoots. Bobb's submodel animation code?
For dying, you need to have another skeletal animation.
Gravity- objects need to fall at a certain rate until they hit another object below them. Messy.
Now for the controls. A 'jump' is a lot like hitting the afterburner once, while sitting still and facing 'up'. That's easy enough - replace the afterburner sound with a grunt, make its duration extremely short and only recharge it once you hit the ground again.
Moving forward is like going to full speed with a really fast acceleration/deceleration time, while you hold some key. Okay, that's doable - again, replace the sound and make full speed only apply when you hold the '\' key.
Moving left and right? Hell, that's already in!
Moving backwards? Duplicate the forward motion. Since FS operates with 'desired velocity' and not 'desired forward speed', it should be as easy as copy-paste and changing a bunch of positives to negatives.
Looking up-down-left-right is a bit harder. The mouse has to be changed to move the eyepoint and gun, not the whole 'ship'.
Make all those changes, and you have a basic FPS.

Anyway, I'd like to see submodel animation code as well, 'twould be awesome to have cutscenes with ingame characters, but it'd be pretty difficult, I imagine.