I really do sincerely think the best way this manga ends is if it allows the MC to life his life the way he wanted until the end. Luffy has constantly said that he will die fighting for what he believes is right.
It's a classic trope, but its one that Oda has mentioned time and time again, even as he has talked about how Luffy is draining his life away using this DF.
I would NOT be surprised if the DF has killed all its previous users this way, including Joyboy.