In an abstract sense, what of a Warhammer 40,000 mod in general? Imaginably, more people are familiar with the planetary Warhammer 40K fighters than the BFG fighters/bombers. Things like landing ops or support missions in an atmosphere might actually be pretty cool... atmospheric combat is already possible for FSO (though I've never played such a mission). If and when we get Nuke's more advanced atmospheric flight modeling, it'll be even better. Also note that all or most Imperial "aircraft" can operate in a vaccum as well, so normal FS missions would work too.
The problem with a support mission is that it would require ground vehicles. SoL probably will adress this, but for the most part I do not believe that this is yet viable with FSO.
As far as BFG goes, does anyone actually know what a Fury or Stormhawk are supposed to look like? I've seen the diminutive game models, but those just don't have the detail to give an accurate impression of what they should look like.
In general, I'd say that a good BFG mod may someday be possible, but it won't happen with the current FSO. UNLESS it's extremely abstract. Think of Windmills: if you do distances like that with a similar interface, you could get off with massive capship battles just because the levels of detail would be so low.
-Thaeris