I will restate some of the things others have said, placing emphasis where I think it is most important.
FSO cannot be made FOSS.
The license restricts it from commercial usage, you would have to talk to volition about reissuing the source as GPL if you wanted to change this (it is conceivable that this could happen, but unlikely).
Even if this happened we would still not be able to make FSO (the game) FOSS, because that would only cover the engine, not the content. Volition doesn't even own the IP rights to such things as the GTD Orion, PVF Anubis, or SB Nephilim, or the IP for the races that these vessels belonged to (Interplay does). Now if you could get the engine GPLed then you could make a FOSS game from it, there are a number of mods out there that have replaced all original artwork and made a completely new story with completely new assets (BTP, BTRL, WCS (all of these use other IP that are not ever going to allow a GPL game, but they prove the technical feasibility of replacing all assets)), but you would not be able to make ANY FreeSpace related game unless you got all the assets and IP GPLed by Interplay (who will not _EVER_ do this). so as nice as having a completely FOSS game based on FSO would be, it is not something we as a community have any control over. we are quite content with the terms of the license so we are not likely to pester Volition to change them and I would find it highly unlikely that they would do so unprompted. If you would like to petition them then no one here would have any problem with that (I would hope), and if you got ANY positive traction with the issue then I'm sure you'd get complete support from the SCP team. we just don't have any motivation to do it our selves because the license currently permits the activities we are interested in, free personal usage. I have seen some recent artwork for some sort of fairly cartoony designed original content that looks like it could make an excellent start for a FOSS FSO based game (assuming we could get the engine GPLed) so if nothing else there is good hope for a free as beer open source game, that is pretty free in terms of liberty (but not completely), but I would not hold your breath for a fully FOSS game to come from FSO, you will unfortunately have to accept some restrictions or look elsewhere, unless
surprises me, which would be nice.