Unfortunately, the game's handling of "cores" in a yes/no manner, rather than as a variable amount, means that it makes no distinction between legitimate claims on territory forcibly taken in a previous war, or land that you merely "want" (as in Italy's claims along the Yugoslavian coast). Borderline, less clear-cut cases, where more than one country has a historical claim, possibly one more legitimate than the other, or where a diverse ethnic mix makes ANY national identity dubious, would be a lot better represented by having up to 3 different claimants, each with some percentage, not necessarily adding up to 100.
For example, two countries could both have solid claims and significant cultural representation with a history of mutual coexistence, so either would be an "approved" owner with over 50% support, or a country could be fragmented and unhappy with either claimant, so neither claimant could have more than a small amount of support. Countries with internal unrest or cultural diversity could have less than 100% claims on their own territory. That would work better than National Unity as gauging local support for the government, and Dissent would be felt in those areas more quickly than in more solidly patriotic regions.
Adding in simplified "pops" from Victoria2 (but only one "pop" per major ethnic group, not individual occupations, etc.), could also allow for finer distinctions between cultural identities from one province or region to another. In conjunction with variable strength Cores, that would probably be sufficient to model the subtleties of China or other areas which have been problematical using the current simplistic system. The building tensions in Europe cannot be sufficiently explained or described by the crude "yes/no" core mechanism, but would make a lot of sense using ethnic pops and conflicting cores.
