I've been reading from a couple of GURPS blogs lately as inspiration for some GURPSified Aliens stuff. Mailanka's Musings has been running a series on making a Star Wars knock-off called Psi-Wars. The series focuses on "filing the serial numbers off" and creating a semi-original work rather than doing a conversion. This gives you the freedom to interpret, steal and change anything you want, as well as pull in ideas from other settings. Another site, Gaming Ballistic, runs a feature called "Reloading Press" where different cartridges are converted into RPG stats. A couple of conversions were done, one on the 10mm Explosive Tip Caseless round from Aliens, as well as a re-evaluation of the M41 Pulse Rifle.
To get me kick-started, I drew up some (low-quality) concepts of a bullpup version (heavily influenced by the IWI Tavor X95) of the M41. I think it looks pretty decent, and is still recognizable as a fill-in for the pulse rifle. I will likely just use some of the ultra tech weapon stats to keep it nice and simple, that's part of the magic of GURPS. Interestingly enough, the ultra-tech assault rifle just happens to be a bull-pup. Lawrence Fishburne doesn't see coincidence, he sees providence.
Now, I don't have a lot of consistent sources to draw from (I've read some of the comics, but not many, and I've watched all the movies) but I get the feeling that the Colonial Marines are heavily controlled by "The Company" (Weyland-Yutani). I also don't want to start researching a whole bunch of third-party, non-canon-fill-in stuff. I'm just going to outright call them Corporate Marines, though I may spiff that organization name up later. For now, I'll keep it simple and somewhat generic.
If the military force is controlled by a corporation, it stands to reason that the lines between government and corporate jurisdictions have begun to blur. This might set us up for a cyberpunk kind of situation with Mega-Corporations running the show (plus there's the Bladerunner/Alien existing in the same universe nods). Maybe Space-X can lead the charge in terraforming efforts.
Aliens never really discusses FTL travel, though it's a given considering the times and distances involved. It's not important to the story in the movie(s), but in an RPG, it may come up. I'll have to think about that part of it. I like the idea of Cryo-sleep, and slow communication (maybe FTL, but still slow, relatively speaking). Alternatively, I could roll with the Expanse and keep everything within our solar system to start. Ripley is born in 2092, so from a technology standpoint, that's not too far off.
I should probably add something here.