I hear there's going to be a Season 5 of Downton Abbey…! (Yea!) And while I don't know what might happen, here's a possible suggestion:
It turns out that Mr. Bates did kill Mr. Green, and he finds out that someone found the train ticket to London from the end of Season 4, which worries him. In fact, it worries him so much that he becomes paranoid, and everything everybody says sounds like an accusation, and so he snaps, killing everyone who looks at him crosswise. Well, this panics Lady Mary, so Mrs. Bates has to kill her to stop her from calling the cops. So, both of them guilty of murder already anyway, they kill everyone else, book a passage aboard a ship to America, and once there, worried they might be discovered if they remain in a large city, they go west, and use their service skills to set up a business outside a small desert town: a motel, named after their surname: Bates Motel. The End.
In this way, if he's tired of making Downton Abbey, the show's writer/creator Julian Fellowes can rid himself of the characters all at once in a way that makes it so nobody will bother him about them again. (They might grouse, but not for too long before they get engrossed in watching some other PBS series.) And if anyone ever wants to know what happens next for the Bates family, he could say they had a son named Norman, and if they want to know more about him, watch the TV series Bates Motel.
