The only two reasons he'd come back at earth would be either to rule everyone, or search for the "more powerful people" that Luffy told him that exist in the blue sea.
Whatever the case, he would be treated as a meme and would only get KO'd immediately by more powerful characters like BM, Rayleigh, Mihawk and others (possibly not Admirals cause then he would have to be jailed).
He is the best at one shot pwn from newbies to Vice Admiral level characters, but i don't remember Oda has ever made a villain that was defeated by Luffy be a villain again, in the sense of "torturing good guys".