What is interesting is where this new culture arose from. Is the high culture of High Rock essentially Imperial (there are sources that support this), with the Celtic anologues being more or less identifiable with the older, Direnni-derived cultures? This doesn't seem to sit well. There seems as much reason for the cosmopolitan style found in most of Tamriel's courts to have arisen in High Rock as in the Niben Bay. It could well be that these did arise in unison with one another, with the wilder peoples of High Rock better understood as a series of infusions of escaped slaves and alike into the wilds; each time reacquainted with old gods. Or at some point, did these people grow into that powerful class that dissolved the Direnni Empire from within?
Imperial and Breton culture have long seemed to be closely tied. Perhaps because they emerge from similar histories? The prevalent theory right now is that both the Imperials and Bretons are actually, primarily, descended from "Nedic" peoples who were indigenous to Tamriel, rather than
just the earliest Nordic settlers (although the tremendous influence of the Nords is un-debatable). They were also both heavily influenced by continental Elven culture, that is, the more material/secular elven cultures of the Ayleid and Dirennis. And ever since the first human empires, they've had a great deal of interaction with each other, so that any differences they might have had may have been diluted somewhat.
There also seemed to be a lot of Bretons in Cyrodiil during Oblivion, and lore-wise there seems to be a lot of Bretons occupying positions in the Imperial government (Although in Skyrim they are again the 2nd most populous group).
Just some thoughts, I suppose. No real conclusion. :\