Ooo, now you're giving me evil ideas for a game. I've always loved spelljammer, but never though of combining it with a modern setting before.
To add a nice horror element, make the evil invaders Illithids, with legions of enthralled slaves who are elves, dwarves, displacer beasts, doppelgangers, and other nasty fantasy creatures. And of course some mindslaves will break free of the aliens and teach some of us poor humans how to use magic...
Throw in a few illithid motherships with elder brain pools, a generous heaping of people having their brains sucked from their heads, and a few random cattle mutilations for no good reason, and it'll be tons of fun.
You could even crop circles a real magical reason for existing. Perhaps they actually are created by people, but aren't hoaxes. They're magical wards that prevent spelljamming and planes-travelling invaders from coming to earth. With the increase in skepticism, fewer and fewer people created crop circle wards, until finally the Illithid invaders were able to break through. Our Heroes(tm) will of course eventually discover this. "We have to save the world! Quick, create some crop circles!" Naturally that won't repel the invaders who are already here, but if created in the nick of time, they may keep out the even worse monstrosities heeding the siren's song calling them to their newly opened ancient homeland: the Great Old Ones.
