This is a writer's dream for a derivative work because there's so very little to get in the way. The setting responds very well to adaption. Look around you; this is GenFS. A thousand different views on everything you consider problematic exist already here, most of themy equally supportable. Those things that are not discussed here are, for the most part, actually irrevelant to a well-plotted story. FS is an in media res sort of world.
What you percieve as weakness is, in fact, strength. Nothing but what you choose to make of it.
A writer's dream, perhaps, but a fan's nightmare. With practically nothing to build on, and what is available is rather generic, the writer gains almost TOTAL control. Their minds go wild and the result is something that says "FREESPACE" in the title, but it won't ever be Freespace.
So to my point: The canon campaigns can NEVER be made into a movie. It's impossible. Side stories within the campaign are possible, but then the writer can go completely off the tracks and create a very generic movie. Even the one I briefly described is something that would be at home as a bad SyFy (god I had that name) movie.
The FS background isn't super special, it's very basic and cliche. The only thing that really stands out are the Shivan's themselves, going by as totally unexplianed for two games and an expansion pack. Most games and movies would never let that happen, but they did it. Everything else is pretty generic. Swap out the models and the names and you could have a dozen different games/movies/TV shows. I suppose you could say that about any game, of course, and that's the point..
As another point, more freedom for a writer often is a detriment to the writer. Limiting yourself when writing, or by given constraints, allows you to fully explore those limitations and truly develop what is possible. Could this be done with Freespace? Not really, because FS has so few constraints that it's not hard at all to break them. Have you ever seen my 14 Year War timeline, the canon version? Aside from a few stated battles and dates, there's NOTHING there. The rest of FS is just as empty. Without a sturdy set of barriers in a writer's way, they'll go nuts, perhaps all over the place and we won't get a Freespace movie, if we're lucky.
Could it be done? Maybe, but damn would it be hard. I'm not hopeful about it. Hell, I wouldn't even attempt it myself.