And if we can get the scripted events for the BL invasions, yes, likely the kind that the Horde and Scourge did.
Invasions from outside the map shouldn't be too hard in CK2. You should be able to work something out based off the Sunset Invasion. Maybe you should talk to the team working on After the End, there's 4 separate invader groups in that.
If you do put invasions in, a Horde invasion and possibly random BL invasions in the late game would be excellent. If you want to model the Plague of Undeath as an invasion, I suppose you could, although something based off of one of the zombie apocalypse mods could work as well.
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