I'm definitely in agreement with the confusion over Cisalpine. In another place of discussion, I pointed out that Venice is, indeed, a fairly distinct population, and if anything could be considered more "italo-byzantine" in common with the far south, more than they could as part of a continuum of the north. Cisalpine culture itself is contentious because some of the lands covered might be considered quintessentially Medieval Italian, definitive of the political systems (commune, republic), identity, art, and political stances broadly (anti-imperial) that we tend to associate with Italy in this time period. However, many areas in northern Italy were particularly influenced by Occitan language, art, culture broadly, in ways that the Tuscans and those further south (as well as the Venetians and Dalmatians) simply were not, and this creep could very much be considered a valid cultural distinction. So, I would personally rate Cisalpine to be a contentious but justifiable group based more or less entirely around how much give or take one is willing to use, where they draw lines in distinction of identity, and a degree of personal preference. The Cisalpine populations might have much more in common with Tuscans than Occitans, but a legitimate gap did form between them in a lot of modes of cultural expression as well.
I still stand by Venetians not being included, though. They should either be plain Italian, or a new "Italo-Byzantine" culture shared with certain places elsewhere in Italy and particularly in the South. Dalmatian also needs to come back, yes yes. I also echo the idea for Rhaeto-Romance, probably shortened as "Rhaetian", being introduced. I'm just glad that they're not using 16th-17th century Italian states and modern Italian regional cultures to determine medieval Italian cultural division, as what we might consider Italian culture should perhaps more properly be considered to balkanize over time, not to unite.
On another note, the Assyrians being subsumed under the Kurds is bad, if not downright offensive to the Assyrian people. There is a significant amount of bad blood both historically and presently between Assyrians and Kurds, it's just entirely inappropriate.
I think the inclusion of a Chuvash culture is interesting but potentially a massive anachronism. There is no universal consensus on the position of the Chuvash people linguistically, but they are commonly held to be direct descendants of the Volga Bulghars, and on a cultural level were almost certainly not distinct in the middle ages even if they are to be considered a separate linguistic grouping by some today. In this region, it is also neat to see the Uralic cultures properly divided along the lines identified in THE OLD TEXTS but I'm not certain of the Biarmian culture. What is it meant to represent? One of the Samoyedic peoples? A Finnic group? Are they really to be considered a distinct group not identifiable as any of their neighboring populations, on the map or otherwise? To cap off the region, I will miss the Goths quite a bit, and hope the Alans there are going to be able to be nomads when that gets reintroduced.
In the Mideast: Should Tajik culture be that big? To my understanding, it's similar to a sort of Persified Sogdian population, so maybe the Khwarezmian culture should be a bit bigger and push the Tajiks closer to traditionally Sogdian lands? Or, maybe, have Tajik be a melting pot for Sogdians under foreign rule, especially under another Iranian culture, or maybe under Muslims? Would also be nice to see an "Aramean" culture as a western counterpart to Assyrian, since there were (and still are!) people who identif(ied/y) as Arameans throughout the Levant, particularly in Syria, more contentiously in Lebanon and the Holy Land under all number of names. Aramean culture could absorb peoples like the Maronites to consolidate itself a bit better, and make its grounding in the whole western Levant more grounded broadly. The region still was largely Aramaic-speaking and even with a large Christian population (potential majority?) until the Crusades or so. The Islamicization and Arabization of the Levant is traditionally associated with the 12th-13th century, and especially around the time of Saladin. Levantine culture could be considered a melting pot of any given Arabic culture (Bedouin, Egyptian, Levantine, maybe even Arab-Berber if that's a thing) and Aramean culture. Getting some South Arabian cultures would be nice, maybe separate "South Arabian" and "Himyarite" cultures, since Himyarite nobles lasted in Yemen well after the rise of Islam and were, linguistically and probably in identification, distinct from the common South Arabian peoples. Whether to split remnant Old South Arabian and the sibling branch of New South Arabian is contentious and I think the cultures might be too small and ultimately too similar to justify doing it, so the real question is which language to pull the names from.
There are also areas in the Levant that, at the 9th or 11th century, would potentially by majority Jewish or Samaritan. The demographic history of this area is difficult to say the least between the Muslim conquest and the rise of the Kingdom of Jerusalem, but there are a few hotspots, and Jewish presence in the land was relatively strong before the First Crusade. This is the period of the Gaonate, of the Masoretes, of schools and pilgrims and sizable settlement of Rabbinic Jews, Karaites, and Samaritans in the land. Although a shadow of their former selves, the communities Four Holy Cities are all candidates - Safed, which largely remains off the record for Muslim history between the 8th and 11th centuries, but which is attested in Jewish literature and records from the period, might be a decent contender. Tiberias is debatable, but possible - at the start of this period it had more than thirty synagogues, which were still known or identifiable in the 12th century when their destruction was recorded by an 8th century earthquake. This implies, at the very least, that knowledge of the synagogues was not lost, the Jewish community there was not simply forgotten or dissipated in that time. Significant Jewish communities are also attested along Gaza and general Philistia, as well as in Ramla-Jaffa, in Acre and Haifa (the latter of which is said to have Jews as its primary defenders during the crusades, implying Muslims were essentially a garrison rather than a majority), and to some degree Jerusalem. The exact population of Jerusalem, like the rest, is pretty hard to find, but I can find one estimate with an actual number given (10,000) for the era, and it is said that 6,000 Jews were slain and more captured during the siege of 1099. This points to a possible Jewish slim majority or plurality in Jerusalem prior to 1099. I have no doubt that more research needs to be conducted in these cases and none of it is helpful without a more detailed map of the area in CK3, but my point stands that, for 867 especially but also 1066, there is real potential for places with majority Israelite culture, and Judaic religions (Rabbinic, Samaritan, Karaite) to match.
For Britain, I find the separation of Cornish and Breton to be interesting. In a lot of ways, this was more of a geographical than a true cultural and linguistic separation during this era. However, for gameplay purposes, it makes a lot of sense - especially with carried-over CK3 equivalent of cultural retinues. The Bretons would be much more likely to engage in French military practices than the Cornish would be, but I hope they can have some sort of affinity for one another at least. I applaud the addition of Cumbric culture and the survival of Pictish into later history, but I can't help but notice "Gaelic" culture distinct from "Irish". Does this perhaps hint at a sort of Old Gaelic proto-culture that will dynamically evolve into Irish and Scottish, and maybe more (Manx? Irish colonies in Wales?) over time? That'd sure be interesting!
For Central Europe, I'm not on a crusade against larger cultures, wanting to break them just for the hell of it. I actually wonder if distinguishing Swabian and Bavarian culture in 867 might not be anachronistic? I'd think it should be united with Bavarian as "Suebi" or "Alemannic", in accordance with the tribes that governed this area, the former lending its name to Swabia, and the latter being the preferred modern term for this area and its culture and people. Franconian is nice, but given its area and what it likely represents, this could probably just be named "German" to no problems. I am surprised Dutch has no presence at all, nor does Frankish - its direct ancestor, and distinct from Franconian.