How about something like an Alien (as in the movies)? You start the game as a little dude just popped out of someone's chest and your goal is to wreak as much havoc as possible in some free-roaming environment like a city or ginormous space ship. The game could even react such that if you don't hide your kills in, say, the sewer your reign of terror will be discovered and eventually you will be hunted by varying degrees of human adversaries beginning with the police and possibly ending with a predator.
Kill some dudes, get bigger, kill more dudes, decide if you want to be sneaky or incur the wrath of local law enforcement, etc.
Kill some dudes, get bigger, kill more dudes, decide if you want to be sneaky or incur the wrath of local law enforcement, etc.