I'd love to see an aliens invading earth board game. Metagaming did a dandy one years and years ago called Invasion of the Air-Eaters. Larry Niven and Jerry Pournelle wrote one of the great alien invasion books in Footfall which could make for a great game.
One example might be that one player, the aliens, would have technological superiority but fewer resources and numbers. The Terrans have to overcome international distrust between nations and come together as a planet to meet the invaders. As the game proceeds, the Terrans' technology develops due to research on captured or stolen alien tech, etc.