i think he rather meant a campaign inthe existing FL universe. a new, different story.
you would need an expert on the misison scripts and a well thought out story and plan, but ti would be possible.
however, i dont know of anyone who seriously tried this.
So you would want a campaign without:
-Cutscenes
-Voice Overs (You can't use Trent, Juni and all the other normal characters again, you'd have to pay to get those voices)
-Good and involving storyline, and no it's not easy to create on of those.
-And you'd probably require New Base, Ship, character etc. models if you don't want it to totally look like a clone of the original storyline but more of a sequelish or at least add on style storyline.
Kabouter, mapping for the Phoenix Mod.
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