The genre of the first movie is horror, and the second focuses more on action because the creature is already known by the audience. I'm not sure about the third, but it seems to focus more on something like detective fiction, as protagonists attempt to outwit the antagonist, with complications that are developed in the fourth movie.
Given that, a fifth movie without any retcon would probably go back to the genre that dominated the second movie, but it should serve as an ending for the franchise. That will mean a new set of protagonists, a new setting (perhaps a large space city where human beings are forced to live as the earth is no long inhabitable due to pollution, etc.), and some backstory about the creatures that will make a confrontation between the aliens and human beings the last one.