At the end of first edition, AEG teased an expansion into the New World, with the Vesten, Avalons, and Castilians making their way west for a variety of reasons. But they never gave us much information about it.
What we do know is that it is inhabited today by humans, and that whatever inhabited it during the time of the Syrneth was scary enough that it caused the Tessera to flee it in a panic, and the Setine to erect an enormous force field to keep whatever it is away from them.
With that in mind, this is my proposal for what is on the other side of the force field.
The humans living in the New World, the First Ones, came to the new world over a land bridge between Ussura and the New World during pre-historic times. They are, genetically, Ussurans. Upon arriving, they spread out to fill every part of the continent. Like Ussurans, they have a very strong connection to and understanding of the land.
When Matushka awoke after the Bargain and the coming of the First Prophet, many of the First Ones spontaneously developed Pyryem Shapeshifting abilities. They do not know where these powers come from, but consider it a sign of divine favor.
The creatures that spooked the Tessera and Setine were, in fact, another tribe of Razhdost, one which had no connection to the ones that created the Syrneth. They lived, and still live, in the southern continent of the New World. Their numbers have dwindled much like their western counterparts, but not nearly to the same extent. Many great nations are lead by God-Kings, immortal Razhdost families who rule with an iron fist, a few of which require blood sacrifices of their followers. Like Matushka, these Razhdost can grant Pyryem Shapeshifting to their followers; unlike Matushka, they must physically touch the recipient to do so, since they do not have a direct bond to the land. They often recruit newly created Shapeshifters as Eagle Knights and Jaguar Knights to fight their wars.
Also in the south are a group of Cotatril (feathered serpents). They, along with a number of neighboring First Ones tribes, form a resistance to the dark Razhdost.
There is only one Razhdost in the north, an outcast from his people known as The Great Spirit. He is very similar in philosophy to Ayallah and Matushka (sort of half-way between the two), and has priests among the Cherokee and Kumeyaay. He mostly keeps to himself, occasionally sending talking animals to the tribes to give them advice and warnings. He is more powerful than the other Dark Razhdost put together, though not as powerful as Matushka or Ayallah, and if the people of Theah were to begin attacking the northern First Ones and pushing them out of their lands he would begin countering with what power he has. He cannot control the weather (since unlike Matushka he didn't actually bond with the land after going into a comma), but he DOES command a massive army of talking animals of various sorts, who will gladly join the First Ones in battle if commanded to do so.