Set up isolated forts and garrisons in the middle of nowhere, spreading our forces more thinly to keep stuff from sneaking in when we're expecting our opponent to forgo subtlety in favour of brute force?
I doubt it'll be more than a minor roadbump against the Centigor Crusade, but fine.
River navy sounds good though. Not sure how Talabecland/Hochland/Middenland would take it, but it could help us scout, harass and deploy against things trying to enter Ostland from our Southern/Southeastern direction.
One of the possible Zacharias Prep options involved putting (vampire-weakening) holy symbols everywhere.
The equivalent of that would be putting more(or better) cannons on the walls of our settlements(especially those inhabited by Hohenzollern, such as Wulfenburg and Vogelsang). Or to somehow fortify the roads and farmlands.
Felfang seems to have been from the Northern Wastes, as he was wearing metal armour(looted from Chaos Warriors) and knew about the Gods of Chaos.
Granted, there were other portals...
Point.
Letting the Witch Hunters and/or the Cult of Sigmar take the fall might lead to something getting done to curb the excesses of religious extremism(or limit the electoral power of the Cult of Sigmar), with all the pros and cons that entails.
Holders of the Shore did that one.
@torroar did the Black Arks' departure change the Raider Pressure(s)?