My proposal is to changing the cores into 3 different values(instead of 1 single value, as it is just yes or no right now)
core=1: This will represent the Homeland provinces, where the whatever culture habitated for a long long time.
-CB for this provinces will cost no badboy
-POP's with the correct culture will be more attacted to the provinces with Homeland core than anywhere else(Hungarians less likely to leave Hungary if immigrating internaly in Austria or Austrians choose to immigrate to Venetia instead of a newly gained Bosnia, for example)
-Core-culture POP's will have huge penalties for assimilation(Poles of Russia, Austria and Prussia)
-Minorities will have higher bonuses for assimilation(while high, it won't be as high as Homeland penalty, obviously. Justification for this is that someone will be more likely to assimilate into German culture in Germany than in Bohemia, for example.)
-It will not be possible to remove this kind of cores in-game, Collision between different contries Homeland cores will always cause trouble between them(Alsace-Lorraine anyone?).
core=2: This is the cores as we know, the territory that is considered a rightful part of the nation in the eyes of it's population and other nations.
-CB for this core will cost a little amount of badboy(For gameplay balance)
-Core-culture POPs will be somewhat attacted to the provinces with this core IF AND ONLY IF they are immigrating internaly(as they are more accepted here than elsewhere)
-Core-culture POPs will have some penalties for assimilation(Germans of Bohemia/Poland in case of their independence, some of them will assimilate back into them but most will resist as they lived there for a considerible amount of time.)
-Minorities won't have any bonus/penalty for assimilation(why would they anyway)
-It will be posible to reduce this kind of core in-game into claims if Primary/Accepted culture POP's becomes minority within the province(which I will explain next)(An independent Poland/Bohemia again, slowly gaining legitimacy and eroding the Prussian/Russian/Austrian legitimacy there)
-Collision between core-core or homeland-core will cause trouble, but if the status-quo remains for a long time, the troubles could cease awas as cores reduced to claims
core=3: This will represent claims in the game, easy to gain but also easy to loose if you don't own them for some time
-CB will cost more badboy than core CB but cost less then outright conquest
-Protectories should be able to gain claim-cores if they have some primary POPs. This fits better for colonies in my opinion
-Claims will promote to standart cores if the province has majority of primary/accepted culture POPs(or something like 30-40%) after a fixed amount of time
-It should offer no bonus or penalty for assimilation
-Claims will be added/removed after a fixed amount of time if you own/don't own the province
Well these are my ideas anyway. Thanks for reading and apologies for my English