From everything I've gathered from the lore Troll Country is a rather large region separating (and buffering) Kislev from Norsca while disgorging plenty of threats of it's own hostile to both sides. It seems odd that aside from some attrition causing swamps (and a Dwarf mission in the area) it functions as a normal part of human settled lands. Azhag the Slaughterer is supposed to come originally from this area (though I understand locating him in the normal Greenskin starting position), yet as things stand the Greenskins have no presence that I've seen at all.
As it is Kislev usually gets largely destroyed by Norsan factions even before Chaos shows up (though in my own games I installed a mod to reduce how quickly they build armies). I don't pretend to have a perfect solution, but making Troll Country a dangerous region not owned by either of those sides might shield Kislev a bit from that side while making the strategic map a little more lore friendly.
There are quite a few other remote areas which probably should not function as parts of provinces, but Troll Country seems the most obvious and changing it would have larger implications.
Total War: Kingdoms lord name Ecbert.