I still like to keep an open mind, even to the malevolent possibilities, but I have a problem with issues like “what could they possibly want?”. Earth has no resources that you can’t find more abundantly somewhere else. Water, gold, both can be found in far greater amounts in the outer solar system and the asteroids. If they want a livable planet, I’d think it would be easier to terraform a nearby planet rather than transport their entire civilization light years across space. It makes no sense to me, but maybe I’m just a stupid human with a puny brain.
Chocolate. Coffee. Cocaine. Kudzu...
Protein.
Man is curious about all things. Perhaps they also are.
Imagine what our neighbors think of us. We are a cruel species that does terrible things to our own kind. Then we develop nuclear weapons and become space faring. That is a sure recipe for attracting attention, don’t you think?
Souls. And they don't need to cross interstellar space. They've been here from the very start.