Cultures are incredibly important in CK 3, even more so now that hybrid cultures and religions are implemented, so the developers have put in some work in optimizing the areas with cultural differences that had some issues.
Starting with a small culture addition in southern Europe, devs have added back a fan favorite from CK2; Carantanian. The culture is quite extensive on game start and covers most of south-eastern Bavaria.
From a historical point of view, the culture is of a West Slavic origin, but as they got cut off from their ancestral brethren in the Carpathian Basin, they became gradually closer to the south Slavic peoples. This is represented by Carantanian having a West Slavic Heritage, but speaking a South Slavic Language.
The culture rework has been a good opportunity to rework the cultures in India a bit. The current setup is one that is inherited from Crusader Kings 2 and it was in some ways not entirely appropriate for the era. To better reflect the diversity of the subcontinent two new cultures have been added, the old ones have been changed a bit, and a number of potential culture names for when the large starting cultures diverge have been added as well.
While looking over the subcontinent it was also clear that in some areas the title setup was also better suited for the early modern era, rather than the medieval era around Crusader Kings 3 start dates. A number of baronies have been renamed and reorganized into new counties, and a number of new vassals have been scripted in, especially for the 1066 start.
The starting presumptions about who controlled what in 1066 have also been revisited to bring things better in line with history and create a more interesting start. There are now more starting characters, both independent and vassals and most kings will no longer start above their domain limit.
Some things like the crisis of the Chola empire should also be a bit better represented in the initial setup, with strong and somewhat unruly Pandya vassals, a much stronger Lankan revolt and the Chera Raja now independent as well.