A Court of Thorns and Roses and Throne of Glass
First things first, I am referring to the Cauldron when I say "the bathtub". I love watching Christine Riccio's videos on YouTube (polandbananasBOOKS), and her affectionate term for the Cauldron has grown on me.
Second, this post will have major spoilers for all books in both of these series, so if you have not read up through both A Court of Wings and Ruin and Empire of Storms, I suggest you do not read this yet unless you want to be spoiled.
Theory: Aeline Galathynius is going to make the bathtub (and the wall and Prison) from ACOTAR. Theses are all things that survive years (thousands, millions, who knows) and become a part of the world that exists in ACOTAR, therefore the ACOTAR world is the TOG world set way into the future. The ancient history of Prythian is the story of Aelin and Erilea.
Aelin has a serious issue of how she's supposed to put the three keys together to form the lock that doubles as portal thing to other worlds. Either her or Dorian is supposed to die to make this happen, which I'm dreading either way. But it would literally be just like Aelin to find some new direct magic or something to find this loophole to make a still usable, but very hard to contain and control, conduit that doesn't require a death. Plus, with the Cauldron the capability to travel to other worlds still lies within the realm of possibility, so Aelin doesn't have to worry about the timing of getting all of the Valg out before she shuts down the portal for good.
Also, look at the details: three keys to form the lock, three legs to the bathtub. Coincidence?
In the process of all of this, Aelin could probably also solve the great debate of fae versus human debate, because Chaol brings up a really great point in that the fae are so muc more powerful tan the humans that there is almost no way to check all of the fae. In response to this then a wall is built to separate the two so that stronger jurisdiction can be put into affect over the fae, securing protection for the humans as much as possible.
So then in the north, where Terrasen is, all of the fae live and over time evolve to live in different groups that correlate with specialties. Specialties that evolve over time as well, becoming the seven different courts of Prythian. Look at the two maps: they are very similar. It would not be too far of a stretch if over time the land changed. The top of Erilea doesn't exist either, so it is entirely possible that the rest of the fae spread onwards and upwards if they don't necessarily like Aelin's rule or whoever comes after her.
The other thing that fits in this theory is the Prison. That thing could have totally been made to contain Maeve.
Another thought just crossed my mind.... the ACOTAR characters could totally be evolved from the TOG fae. Lorcan is blessed by the god of death, which could eventually translate into the Night Court. Rhys is called "Death Incarnate" at one point by Feyre, and who has been blessed by the god of death? Lorcan. Also- now this is really a stretch- without Cairn, there are five members of Maeve's cadre (I'm not counting Cairn because I'm really hoping he doesn't make it through the last TOG book). Throw in Aelin and Aedion and you have seven differnt major fae figures..... and there are seven courts in Prythian. OH! Throw Cairn back into the mix and his bloodline can become the King of Hybern.
All of this is stemming from the fact that Feyre and Rhys have to fix the bathtub because Amren broke it. Amren is definitely a goddess of some sort that was trapped in a fae body. The description just fits perfectly to what we know about the TOG world and Sarah's writing. And we already know that those gods and goddesses can take over a fae body because Diana took over Aelin's body in Empire of Storms. WHAT IF AMREN IS MAB!!!!! That was random, but think about it. Aelin is blessed by Diana, but a descendant of Mab, who we know ships Rowaelin because Rowan prayed to her in Heir of Fire and got his prayer answered. She would totally then come through to help out Aelin make the bathtub and survive so that she could live with her mate, Rowan. It all fits.
I'm actually really hoping that at least a fraction of this post is correct now, if not all of it, because it makes sense to me. And look at the quotes from the series! I mean really, "The world will be saved and remade by the dreamers". Remade. I think this may be it. But I could also be really wrong. Feel free to ask questions or comment because I would love to flesh out this theory some more!!
Please let me know what you think and thanks for reading!!