A development update on Crusader Kings 3 (PCG's strategy game of 2020) details some of the changes coming in the game's 1.3 update, with the most major being the introduction of Winter to your holy wars. The season will come in three different varieties of harshness, depending on geography, and affect armies in a wide variety of ways.
The harsher the winter, the more snow, and the more supplies any army will use over time. The snow will slow movement, and at times snowstorms will make conditions even worse, in addition to which any battles will be more deadly for both sides.
The largest impact will be on cavalry units, and particularly heavy cavalry which will receive heavy penalties. Fighting on mountainous terrain with horses during winter: bad idea. Other kinds of troop will receive bonuses while fighting in winter, with the example given being the Tibetan unit Mountaineers: "Accustomed to the cold and the snow in the mountains, they can utilize it to their advantage in the form of a damage bonus whenever they participate in battles when winter is present."
In addition to this, the 1.3 update will be re-jigging some existing territories, including the Emerald Isle. "The baronies in Ireland were a bit too large," apparently, so it had "significantly less holding density than its neighbors, England and Scotland. The western half of Ireland in particular suffered from a few quite large baronies. To solve this issue, we’ve added a number of new baronies, along with a couple of new counties, that should improve the overall experience of playing in Ireland."