Great Dev. Diary, King, Victoria2 sounds each time more promissing!
This system looks like quite perfect for the time scope of this game, as it handles very well with both the unifications and secessions that occured (or could have occured).
You said about the three cultures inside the American Culture Group, and I was thinking you could do something similar to Latin America. More specifically, there could be a Brazilian Culture Group, with Brazil as union tag and the cultures:
-Gaucho, in southern Brazil, that revolted as the Piratini or Rio Grandense Republic at the start of the Victoria timeframe; (to not overlap cultures, the "gauchos" in spanish regions could be simply called "platineans", IMHO).
-Nordestino, from northeastern Brazil, much more favourable to slavery and other conservative views. They were the "agricultural power" and most "precious" part of Brazil in the colonial times, but became decadent after that, exactly like the Dixies in comparison with the yankees (except that the Dixies were in the South and the Yankees in the north, and in Brazil it was the reverse). They rebelled a little before the Victorial timeframe too, with the Confederação do Equador in 1824 - and there were many other smaller revolts within the XIX century, that could be taken, in game, as revolts of the Nordestinos pops rather than hard-coded events, as they were in Vicky1 (or was it in VIP?).
-Brazilian itself, or maybe Sulista (while they weren't the southernmost region, sometimes the Nordestinos call the Southeastern ones "Southerns" of Sulistas). It would not be fair to name then after one of the states of the region, though. They, as the yankees, did not have a separatist movement, but, if needed, one could use the Inconfidência Mineira, from 1789, as a base for an separatist Sulista state - its symbol, a red triangle with the motto "Libertas Quae Sera Tamen" (Freedom, even if delayed) would be certainly used if they wanted to succeed.
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Another thing I think would be interesting is at least one unifier state for each culture in a Culture Group. For example, I always missed a Rhinelander Nation to be formed as a puppet state when playing with France and conquering that area. I don't know if - or how - it would be historical, but the creation of a unified puppet state would be really plausible in this situation. By the way, would "Rhinelander" be a culture inside the German Culture Group?