Hammerfell and High Rock have had quite a storied history together, from the Yokudan Ra Gada conquering the Deathland, including the then-Breton port, to the various wars that took and alliances that were formed between the Bretic and Redguard kingdoms, to both High Rock's and Hammerfell's integration into the Empire. But now, with Hammerfell's secession from the Empire, the matter of where the border between the two lies is of some importance.
In Daggerfall and ESO, we are given two very different border demartations, the former, chronolically more recent example, has the Bjoulsae River essentially serve as the border, while the latter has Evermore control a large part of the river's southern bank, with Bangkorai Pass, a mountain pass leading through the Dragontail Mountains, serving as the border. I think we could very well see a variant of ESO's demarcation carry over to TES VI, in large part because Bangkorai Garrison, a fort that spans across the Bangkorai Pass, could serve as an immersive barrier to block movement between the provinces, as well as potentially an access point for DLC.
There is however the question of who'd hold Bangkorai Garrison in the Fourth Era? Evermore's Order of Saint Pelin, as seen near a millenia before during the events of ESO? The Imperial Legion? Could the inability of the latter due to manpower shortages mean the Cyrodilic administration kicking guarding the border to the former? And what could that entail?
We could very see a scenario, where Bretic royals, the rulers of Evermore for example, are apponted governors in exchange for using their own manpower and resources to safeguard the province's border with Hammerfell. And that could lead to even more serious problems down the line, like those same royals turned governors, invested with Imperial authority, beginning to harbor ambitions for the Ruby Throne...
