How is this for an idea. Anybody that has played Medarot will know, but Alien is a "type" within that series that could lend itself to Pokèmon. There exist a handful of species that could be retyped to fit with the Alien type, such as Deoxys, Minior, all the Ultra Beasts. It could be supereffective against steel or some shit, since alien tech or power would be beyond mortal technology, and perhaps weak to fire as a reference to the Alien movies.