I was planning on the first aliens be very reminiscent of the old 1950s-esque aliens, they would be grey aliens with big heads, big black eyes and tiny bodies, they would carry around rayguns, fly around in saucers, abduct people, all that cheesy stuff. But then as a twist, after spending a lot of the story focusing on the first group of aliens, I was planning on having ANOTHER, completely separate species of aliens appear, ones that would be reminiscent of somewhat more modern aliens, like the ones you'd see in a FPS. They would be a warrior race type of aliens who are ALSO trying to invade the planet. The idea would be that the original grey aliens were invading humans because they're trying to steal the resources of our planet, while the later aliens want to use the planet as a home base and as an extension of their empire. So now you have a three way battle: the warrior aliens want to stop the grey aliens from taking away all the valuable resources from their potential future home base, the grey aliens want to stop the warrior aliens from trying to stop them, and the humans just hate both sides and want to be left alone in peace