Please correct me if I’m wrong, but an idea I’ve never read about in any science fiction is the reversal of the classic concept of a colony ship.
The basic premise resides around a race of intelligent extra-terrestrials that have launched their first intergalactic expedition. These aliens have roughly the same level of technology as humans, a little more advanced in some places, a little behind in others, but have geared their country/planet to exploring space much like the United States when Kennedy announced that we Americans would be the first people on the moon.
Now imagine a civilization that after reaching their moon didn’t stop there and instead continued to push onward. Soon after they hit several of their solar system’s planets and then, after years of planning and toil, launch their first colony ship at what they determine to be the closest planet that can sustain their form of life.
Earth.
The colony ship takes over a hundred years to reach earth using some specialized deep space engines. Upon arrival, in our present day, they find the planet they hoped to claim as the first intergalactic colony of their race/country turns out to be inhabited by us. Earthlings. Humans.
Natives.