We've already seen the Predator in a COD game, and it will finally make a long overdue appearance in a fighting game, a Mortal Kombat one alongside Jason freakin' Voorhees on top of that, so what other series would you like to see either creatures appear in? You can also mention the Engineer.
All of them as secret bosses in Metroid, Resident Evil, Gears of War and Metal Gear Solid, just to name a few. In MGS' case, there could be an extra Alien-related mission where hiding is impossible.
The Predator as a secret boss in Assassin's Creed, Tomb Raider, Legend of Zelda, again, just to name a few, and Mirror's Edge. I think being hunted by a Predator in a ME game and trying to flee from it would be a lot of fun.
(Imagine a LOZ game, with the classic overhead gameplay, and the screen suddenly becoming infrared, followed by the Predator locking on to Link, and players having to dodge plasma bolts until the Predator decides to take them on, at which point the screen returns to normal.)
The Predator and the Engineer as playable characters in a F-Zero game, and their vehicles being miniature versions of their respective ships, and the Engineer wearing it's suit when driving.
A resized Queen in a Rampage World Tour sequel, and a Primal Rage sequel/reboot. Speaking of gory fighting games like PR and MK, I personally wouldn't mind the Aliens' acidic blood not factoring in their gameplay.
The thing is, I'd be okay if we fought the creatures in new Alien/Predator/AVP games that had similar gameplay to some of the aforementioned games, instead of them making cameos in those games.
