I found John Harper's outstanding Colonial Marines hack for Apocalypse World, THE REGIMENT, which I highly recommend to anyone who likes that ruleset. That was excellent, but a bit too crunchy for what I've got in mind. There's also Gregor Hutton's sublime storygame 3:16 CARNAGE AMONG THE STARS, which appends clever character-building through flashbacks onto dead-simple mechanics. Perhaps a bit too simple, but I haven't entirely given up on it.
In retrospect, it's amazing that over the years there have been so few games that tackled the "bug hunt" niche, although maybe the truth is that movie licenses are expensive and most people just come up with their own hack as needed. I can think of the Amazing Engine supplement BUGHUNTERS, and the official ALIENS game (I think that was late 80s / early 90s) from the same guys who did Phoenix Command. That one was likely too crunchy for me. Of course, there's always SPACE HULK, though I never had the means for that (despite loving GW's games), and the tabletop 40K space marines games.
I found that ideas for an Aliens-themed game came easily, and the world of the "xenoverse" seems closer than ever these days. Modern soldiers are beginning to field equipment on the battlefield that looks an awful lot like a Colonial Marine's kit, and drone weapons are eerily reminiscent of the sentry guns (for those of you who have seen the extended cut of ALIENS). Weyland-Yutani only looks more and more like a modern multinational (which is to say, Big Evil) corporation, much like a spacefaring descendent of Halliburton. I've seen technical demonstrations of devices that look an awful lot like early runs at the big yellow Power Loader that Ripley uses to lay a whuppin' on the xenomorph Queen.
What sorts of things would Marines do when they weren't dealing with bugs, I wondered? Pacifying breakaway colonies and political uprisings seems likely, and I can easily picture religious factions and anti-corporate groups being a factor in a spacefaring culture where huge megacorporations are so chummy with the government (for all we know, they are the defacto government).
I also started imagining an enemy faction that descends from rogue androids, the Meks. (It's easy enough for your thinking to end up on killer robots when you start down the James Cameron primrose path.) These would be androids who shrugged off their programming, much like the replicants of BLADE RUNNER, to create very dangerous terrorist cells. The first generation Meks ritually remove their skin, to reject the appearance of their human oppressors. Second generation Meks are custom-grown to look non-human, with an appearance much closer to the "greys" of UFO lore. Third generation Meks are entirely robotic, with some looking very Terminator-esque and others looking entirely non-human.
A galaxy full of threats like this, in addition to the usual creepy crawlies and corporate goons, could present a lot of play opportunities.
Who wants to strap on a smartgun and jump in the dropship? Your express elevator to hell awaits.