Immersion killers are situations that for me, just appear unrealistic or downright strange. While obviously CK2 isn't a perfect historical simulator and doesn't try to be, I think there are a few issues which could be improved upon, for gameplay reasons as well as historicity.
One of these is the accumulation of territories which seems to happen over the course of the game so that by the late game a few powers seem to control counties all over the map. In my current game as Lombardy by the late 1300s Western Europe had pretty much been divided up between Ireland, the HRE, England, and Castille. HRE owned their core territories of Germany, Bohemia and the low countries as well as all of England; Ireland owned Scotland and a large amount of France such as Aquitaine and Toulouse; Castille owned the Iberian Peninsula as well as all of Ireland and England owned a couple of counties in England but their main power base was in Southern Italy. Another strange situation is Georgia owning a huge amount of land in northern Russia and Scandinavia, which is separated from its home territories by the Golden Horde.
It made the entire map disjointed and confusing, with the majority of rulers having their holdings split up all over the map and the weird thing was the relocation of entire nations to new places, e.g. the fact that "Ireland" no longer controlled any of its original island but was now a major power in France. I know that historically rulers did have their possessions split up, such as Spain's possession of the Netherlands in the (1500s?), but that was generally the exception not the rule, and possessions which were removed from their kings by a large amount of land and with foreign powers cutting through the middle of them generally split off and became independent sooner or later, which I haven't seen happening in CK2.
Maybe a solution to this problem could be to increase the relations penalty from distance for vassals, or increase the 'foreigner' relations penalty. I don't know what the answer is, but it's quite a strange situation in game as it stands at the moment.
One of these is the accumulation of territories which seems to happen over the course of the game so that by the late game a few powers seem to control counties all over the map. In my current game as Lombardy by the late 1300s Western Europe had pretty much been divided up between Ireland, the HRE, England, and Castille. HRE owned their core territories of Germany, Bohemia and the low countries as well as all of England; Ireland owned Scotland and a large amount of France such as Aquitaine and Toulouse; Castille owned the Iberian Peninsula as well as all of Ireland and England owned a couple of counties in England but their main power base was in Southern Italy. Another strange situation is Georgia owning a huge amount of land in northern Russia and Scandinavia, which is separated from its home territories by the Golden Horde.
It made the entire map disjointed and confusing, with the majority of rulers having their holdings split up all over the map and the weird thing was the relocation of entire nations to new places, e.g. the fact that "Ireland" no longer controlled any of its original island but was now a major power in France. I know that historically rulers did have their possessions split up, such as Spain's possession of the Netherlands in the (1500s?), but that was generally the exception not the rule, and possessions which were removed from their kings by a large amount of land and with foreign powers cutting through the middle of them generally split off and became independent sooner or later, which I haven't seen happening in CK2.
Maybe a solution to this problem could be to increase the relations penalty from distance for vassals, or increase the 'foreigner' relations penalty. I don't know what the answer is, but it's quite a strange situation in game as it stands at the moment.