This theory was bogleing my head for some time. With the question why was Subaru transported there? No one knows if it was the Witch of Envy or some other reason. I've read past vol 4 and Al is also affected this, but i don't think he knows if it was the witch's doing(yes i'm that far behind of the LN, don't judge me.). So i was thinking, what if it has something to do with the past Kings of Luganica, or involved with the other witches? Because i was thinking what if the past kings has been reincarnated into the modern world with serviere memory loss and transported back to the fantasy world because of the dragon's covenant. Due to fact of the dragon's blood?
And this is where the 2nd question happens: is there more people than Subaru and Al? As mentioned by Priscilla, they were talking about something about "The Great Waterfall". Does that mean it's a portal that connects both the modern and the fantasy world? If so, then does that means there are other people other than Subaru and Al that came before and/or after? And if so... does this affect the cult or the demi-humans too? Because I thought of something as i read the volume. And i thought of some prime suspects: Otto, Ricardo, Regulus, and/or Elsa. Because i've seemed to notice there's a familiarity what they all have in common with Subaru & Al, and that is "The Equivalent of Exchange". Like Al, he lost his arm when he 1st appeared in that world. So maybe, what if by example: Regulus was given the power of greed when he 1st transported, or Ricardo was actually human in the modern world but exchange his humanity to enter the fantasy world. I think thats what i thought in this theory, and i got another theory about it, but i think this alone would be suffice.