Am I the only one who half-suspect/hope that TaleWorlds will be bringing the nord invasion of Calradia as an end-game disaster? Personally, I'd love it if nords behaved like an improved version of the vikings in Viking Conquest. In the early game, they'd limit themselves to the occasional village raid along the coast, while in the mid-game, larger hosts would appear, hoping to settle new lands or even seize undefended villages or castles along the shoreline. As time goes on, their presence could gradually grow until a large invasion fleet tries to carve out a kingdom or two for themselves.
It would be pretty difficult to manage this without some noticeably heavy railroading. The late-game mega-factions that tend to form are very, very difficult to take fiefs from. High prosperity plus food improvements to the settlements mean huge militia and garrison numbers. Absorbing lords from dead and dying factions gives them massive field strength (20,000+). They are rich enough to never care about money. Fief improvements, serving in armies, battle gains, kingdom policies, etc. all mean they have stacks of influence as well.
And if anyone does take a town or castle, it gets sieged as soon as an army arrives, while equally large armies show up to raid the bound villages flat. If the first army fails, a second army steps right on up and does the same thing. Then a third. Then a fourth. Then a fifth.And since autoresolved battles always cut reasonably close to even, both sides are taking huge losses, but the mega-faction can just strip its (hundreds strong) garrisons for instant replenishment.
To pull off a Nord invasion against that, you'd need several doomstacks, simultaneously sieging more than five settlements (five is about the average number of large armies a mega-faction will have in the late game), along with raiding. And even then I'm not sure they'd win because the current diplomacy leaves plenty of time for peacetime build-ups to all or mostly high-tier parties. Or just cheat in some other way.
Perhaps gifting the invading Nords a pseudo-city "landing site" that allows for fast and plentiful replenishment and won't ever be targeted by the AI?