My approach to a lot of these games is from that of humor and whimsy, so I offer many of these not as hard combat games, but with light role-play and much laughing in mind. Also, the aliens might not always be the aggressor.
Capture or stop the capture of the human(s).
Stop the aliens without killing them.
Capture the alien.
Aliens stop for repairs, just trying to get what they need (bumble bees, a vacuum cleaner, whatever) and get out of here.
MIB v. KGB sleeper cell starring in …Get to the Aliens First.
Two groups of aliens, one trying to stop the other from hurting/interfering with the humans, while humans try to kill them all.
Aliens stop for cocaine, battle the drug lord, street gang, crime boss, etc.
Aliens here to capture a puppy for their leader, good old boys want their dog back.
Aliens stop for booze (could be something silly like some cola, motor oil, something not obvious).
Steal the flying saucer. Could be the army, bunch of rednecks, groups of boy scouts, etc.
Add in the MIB interfering as they might to any scenario to magnify the chaos.