I still feel like it doesn't make much sense for the gate to lead to Earth, otherwise border patrol would have it under a lot more security than just a velvet rope, unless maybe they're going for a "hide it in plain sight approach." That being said, one possibility I've been mulling over is that maybe it's something like a magical replica of the human world, and there's something of Delckira's in it that Ali is interested in.
It'd give us a pseudo Earth arc without completely committing to leaving a chunk of the cast stranded in the human realm (something that I feel like we'd need more build up for since there isn't much plot to be had from the characters just simply being stranded on Earth.)
It is an ancient artifact that belonged to the demon lord and seemingly can’t be opened. Even if it once led to the human world, that kinda knowledge could’ve been lost over the centuries and now it’s just an old artifact. Plus, I have a theory that only a human can open it.
as for the earth arc, I’m not sure. I was hoping we’d get the whole trio there for it and that’d be the big reveal of Iruma s secret (maybe him even losing his memories of the demon world and going back to being his parents slave until they rescued him) But I don’t think the other 2 are close enough to make that happen.
however, on the other hand, Ameri already knows his secret so her going with him wouldn’t be as impactful. Plus there’s a theory that the creator of the manga they love is a demon and specifically Ameri’s mom. Now that’d be an interesting reunion and could kick their relationship up a bit if they got the mom’s approval.