The in between period is called the 'interregnum' because the Akaviri Potentate originally positioned itself as a temporary regency pending the discovery of a new dragonborn emperor, and because Tiber Septim leaned on the Reman empire for legitimacy, not because each warlord saw themselves as trying to become emperor.

The various warring states were not attempting to recreate the Reman empire; in general they were just returning to the status quo before Reman united most of the continent against the Akaviri invaders (who then swore fealty to him). When I wrote 'nominal' control I meant:

Reman troops on the coast of black marsh with most of the coastal argonians swearing some form of fealty on pain of having their hist trees burnt to the ground

Alinor officially being subservient, but even Reman diplomats not allowed outside the diplomatic district of the capital

Half or more of high rock, hammerfell and skyrim being de-facto independent (reachfolk, orcs, alikr)

An unknown portion of the jungles of cyrodiil filled by ayleids still

The Septims and Remans are entirely seperate entities with entirely separate organizational structures (as far as we know anyways; we have mostly just myth/song/legend for the Reman period).

Setting aside Reman empire bc there's a distinct lack of information on it, we have indication in Morrowind of the empire's relationship to the province it has the least official control over. So imo at least this tells us that the provinces outright conquered and forcefully integrated would have even more unequal trade relationships (and e.g. in Argonian account we hear of slavery in Black Marsh, of Argonians, by imperial immigrant-landlords).

exploitative core irl had new frontiers for a capitalistic expansion, but Tamriel doesn't have new anything new for many centuries besides Akavir which they don't successfully exploit. so people itt are postulating some sort of medieval neoliberal turnabout where a core territory of the Empire (Skyrim) is a peripheral subject. much simpler to just think of it as medieval China or Holy Roman Empire

I wanna re-iterate my position that Skyrim is part of the Septim/Mede Imperial Core; i have no interest in arguing about what others in the thread are arguing.

"Tamriel" is a continent. The Empire is based in the human core of Cyrodiil, 1/2 of High Rock and Skyrim. The Imperial Core of the Human Empire has tons of New Frontiers to expand into; Black Marsh, Morrowind (Temple preserve of Vvardenfel newly open for business and settlement in Morrowind), Valenwood, the Orc controlled territories of High Rock (eventually taken over through co-optation), the goblin controlled territories of Cyrodiil, any area considered 'wild' and therefore being "unproductive" (given time, this would likely see the empire drawn into conflict with the Bosmer bc green pact, but thats part of why the Septim empire pushes the cult of the 8 and 1 so much).

And you mention the HRE, but it should be noted there was a whole frontier (both for the feudal lords and for the bishops) to the East of it for the entirety of its existence. There was continual economic pressure put on them, German merchants (and bishops) and outright settlers throughout Hungary, Poland, the Baltic. When the HRE was created, like half of Germany wasn't even German or Christian and that would change within 300 years.