Here's some of my thoughts revolving around factions that could keep with the setting of the late-Roman/early-Byzantine period:
Darshi: Definitely should be added. They already have a minor faction (the Ghilman clan), have terrain features through their side of the map, and would fit as a stand-in for the Sasanian Empire. If war elephants are too much, I'd have no problem with them being a blend of the Imperial/Khuzait/Aserai fighting styles with cataphracts and horse archers of their own.
Nords/Jumne: Can see them getting added to the massive peninsula above Sturgia. Even if there's no sea travel, adding ferry points for them to interact with the Sturgians would be nice, or adding in more minor clans of Nordic mercenaries that you can hire.
Greeks: Ancient Greece makes no sense with the setting of the base game, and the Empire already represents the Byzantine Greeks of this period.
China: Would either add way too much complexity or be significantly overpowered, but maybe a minor Chinese kingdom or sinicized Khanate like the Guiyi Circuit or Liao Empire. If we follow the early-Byzantine/late-Roman period of the setting, a Guiyi or Western Protectorate-style kingdom would fit right in without breaking the faction balance.
That would be interesting. I'm not familiar with how ancient naval warfare was conducted except a little bit of China thanks to my love of Romance of the Three Kingdoms, so it'd be all new territory for me!
As for the Vlandians, I believe the implication is they come from the mountains around Ocs Hall and expanded westward from there. They may be related to the Battanians, to some degree at least.
I'm curious if he or anybody knows what workshop levels do and how to raise them. I get the impression they'll remain level 1 forever lol, or that it's an unfinished gameplay mechanic that may be scrapped since a serious problem with workshops is inundating the marketplace so higher grades maybe ought to produce higher quality (but higher price) goods to avoid devaluing the stuff that's already in the game.
Vlandians are supposed to have sailed to Calradia, and Ocs Hall didn't become Vlandian until after they rebelled from the Empire (which happened only four generations before Bannerlord, seeing that the first Vlandian king, Osric Iron Arm, is the great-grandfather of Derthert). Ostican and Rovalt are supposed to have been the first Vlandian settlements, with the first Vlandians arriving in Calradia as mercenaries in service of the Empire.
If interested, here's a map of Calradia before Bannerlord assembled using the in-game lore descriptions for cities and villages. The only thing on here that's not mentioned in-game or in the lore descriptors is the Great Liang, which is supposed to represent the Chinese Tang Dynasty that defeated the Gokturk Khaganate and pushed them westwards towards Byzantium and Persia (just as how the Khuzaits were pushed westward by a "distant conquerer").