It's true that Descent and Freespace are from the same editor, and the player controls a ship in these 2 games, that's a common point.
But, Freespace still Freespace and Descent still Descent ; I cannot imagine playing a Descent level with the flying controls of Freespace.. in Descent you have to dodge missiles, and you cannot just accelerate or brake, you have to move left, right, up, down, etc.. because Descent's maps are tortuous and so you don't have in Descent the same liberty degrees as in Freespace. So, if someone wants to bring up a Descent mod for freespace, he would have to make these things possible.
Another hard thing is that in Descent there are powerups, and they are crucial to the gameplay ; so developers would have to make up some item system, plus other keys to switch items or things like that ? (and a different HUD, because the Freespace HUD has been designed for a vessel to cary no more than 4 primary or secondary weapons ; if a vessel cary more, it will be some kind difficult to switch weapons.
Bringing Descent to Freespace would be a really huge project I think. And the gameplay between Freespace and Descent is very different..o
