As the title says, this is quite the crackpot theory, so it may be easily disproven - if so, disprove away. It's just a fun idea that I'm currently entertaining to see what pieces match and which ones do not. I posted this on reddit earlier but was curious what people in here thought as well. tl;dr at the bottom.
For those unaware, the sidekick Rekke you can find in this game comes from a land called Yezuha, located East of the Deadfire Archipelago beyond the eternal storms that hide Ukaizo. He speaks an unknown language and is a missionary of the singular god of his people.
What if the introduction of this new land is Obsidian's attempt to merge the universes of both Pillars of Eternity and their Tyranny franchise, and Yezuha is what became of the world of Tyranny (or is a part of it)?
Link #1: God
The world of Tyranny, at least in the games, is ruled by a singular almighty overlord. This would be the god Rekke speaks of, whether that is Kyros or perhaps their successor, maybe even the Archon of Spires. There are no other gods on Terratus, only the Archons who seem to draw power from belief, but don't claim to be divinities, and all are subservient to Kyros.
The big question that arises from the existence of a singular deity is obvious: The whole reason the Engwithans created their own gods was because they determined that there wasn't one, which conflicts with Rekke's belief. There are any number of possible explanations for that, but one is that Rekke's god is perhaps not the kind of god Engwith was looking for, but a seemingly almighty overlord who grew into power.
Link #2: Ancient Structures
Both Pillars of Eternity and Tyranny feature mysterious ruins built by an advanced civilization. Engwith in PoE is a major plot element, as are the Oldwalls and the Spires in Tyranny. However, the origin of those is never explained, only that they hold great mysteries and power. The Spires may in fact be the origin of Kyros' power.
Let's fill this (current) hole in lore with our own explanation: The Oldwalls and the Spires in Tyranny were built by the Engwithans, who may have originated in Terratus rather than Eora, but then left their home for the world we know from Pillars of Eternity. The magical structures they left behind would eventually empower Kyros to conquer Terratus and declare themselves as Overlord. Considering the feats we know the Engwithans were capable of, this seems to be entirely within the realm of possibility.
Link #3: Storms
Yezuha is the land beyond the storms, and Rekke has the unique sub-race of Stormfolk. Tyranny also prominently features magical storms. The Edict of Storms is the first Edict that Kyros spoke and marks the beginning of Tyranny's calendar. Another Edict of Storms still ravages the country of Stalwart over 400 years later.
Now, in PoE2 we find out that the storm around Ukaizo were deliberately created by Ondra to hide the city. The same storms also ravage Rauatai and, which is important for this crackpot theory, HIDE YEZUHA FROM THE REST OF THE WORLD. Why would this be the case? Given that Ondra did everything in her power to erase the past, obscuring the origin of the gods and of the Wheel, why not obscure the origin of the Engwithans as well? Thus we have a magical storm dividing Eora and Terratus/Yezuha.
Link #4: Moons
One final peculiarity I wanted to point out were the moons in the two settings. Eora is orbited by two moons - the prominent moon Belafa and a much smaller, irregular satellite called Cawldha. An in-game book tells us that if the orbits of the two align, it wreaks havoc on the tides and weather everywhere.
Meanwhile, Tyranny also features two moons - the larger Terratus Grave and the much smaller Interloper. The people of Terratus draw magic from these moons, but we are told that if the Interloper's orbit puts it in front of Terratus Grave, trying to use magic drawn from the latter will result in catastrophe.
Those two concepts sound AWFULLY similar to me. Now, we also know that Eora was previously circled by a third moon, Ionni Brath, that Ondra called down the remains of the Engwithan civilization. This simply means that the third moon was cast from the skies long before Tyranny begins, which would fit neatly into the timeline - the Engwithans leave Terratus, create the gods, Ondra kills Abydon with the third moon, and the people of Terratus eventually forget about its existence, as did the Eorans.
Conclusion: The timeline
So ASSUMING all of this is true, what about the rest of the timeline? The world of Tyranny is not nearly as advanced as that of PoE. This might be explained by Terratus being secluded from the rest of the world and maybe even crippled after losing their brightest minds when they left for Eora. However, Rekke expressed no particular surprise at our technological marvels, so I would imagine that Yezuha is not that far behind the Deadfire, at least.
We know that several millennia passed between the creation of the gods and the time of Pillars of Eternity, so if we assume that Tyranny takes place only a short time (relatively speaking, maybe just one millennium) after the Engwithans left Terratus, they would have had plenty of time to catch up. This would also mean that “God” has ruled not just for a little over 400 years, but for thousands. Again, whether that is Kyros or perhaps the Archon of the Spires that overthrew them, staying in power for that long, it wouldn’t be too out there to make the jump from being the Overlord to simply being revered as God.
tl; dr - I’m entertaining the idea here that Yezuha, Rekke’s homeland beyond the storms, is actually Terratus, the world from Tyranny (or a part of it). Rekke’s god is the Overlord, Kyros, or perhaps someone that usurped them, like the Archon of Spires. The Engwithans originally came from Terratus and built the Oldwalls and Spires there, and Ondra’s storm is in part meant to hide that fact. The number and the nature of the moons in both settings serve as a potential link connecting them.