Personally, I think any potential "Alien Invasion" will probably be saved for a One Piece Sequel series, rather than something that ever happens in the main story. I think it's more likely that any final villains will likely come from within rather than without, in this particular story. Aliens will probably be teased as a sequel hook, but probably won't play a big part in the finale of the manga. I could see the equivalent to Boruto or Dragon Ball Super doing the whole "space pirate" thing.
That being said, I do think it's possible that the real "aliens" in the One Piece World could actually be humanity itself. I have a pet theory that humanity is not native to this planet, and that they colonized and terraformed a alien world in the distant past. I think humanity basically "stole" this planet away from it's native inhabitants and kept it for themselves. But, that's my pet theory. We'll see if that plays out or not.
But, there's another theory I'm kind of toying with. I don't know if I'm ever going to write it or not, and I'm kind of waiting to see what happens next chapter a little. But, I'm starting to wonder if the Moon is even really in space or not? It's weird that Enel can breathe on the Moon. We've all kind of just shrugged and accepted that since this is One Piece and weird stuff just happens sometimes, but I'm wondering if that was a mistake. And I'm wondering if there's a parallel with the artifical Lego Land in Elbaf and the entire One Piece World. Just how much of the One Piece World IS artificial, anyway? We know from the Red Line and the Contient Pullers like Oars that SOME parts of this planet is likely artificial. But, just how much? It's something that I'm letting stew for now.
