There is a random Uprisings event where elites of the knightly orders will band together in crusade against you, increasingly likely if you are Chaos and are becoming too powerful. They can have superior equipment that advances with increasing length of the game, they don't come from barb settlements and don't care if you aren't prepared against them, they are meant as a potentially serious threat once you're pulling ahead (it sounds like you were in 1st place and thought you had it made.. around the time I usually stop playing from lack of challenge.) So yes, Chaos got some particularly nasty surprises
But then so does everyone else.. Since the Uprising events don't own any cities or produce any troops, they can't be weaker or the same strength as the fortified troops around them or they wouldn't pose any serious threat.
Re your second point yes on all counts - in moving the mod to the standard codebase of Fall Further we plan to take advantage of the capability for multiple "barbarian" civs. AFAIK the current planned barb factions are:
Feral Greenskins
Leaders with the Greenskin Trait start at peace with the Feral Greenskins; this truce will dissolve if the civilization grows too powerful/civilized but may be regained with a powerful WAAGH! Ritual.
Chaos Undivided
Demons, Chaos Spawns, Marauder Warbands, etc that owe no allegiance to a particular Chaotic faction. Become increasingly more prevalent when the Winds of Chaos counter (Armageddon Counter replacement) is high. Because of the bitter infighting among minions of Chaos these can be a real threat even to the Hung and the Kurgan. Being elected Everchosen of Chaos (head of the Undercouncil replacement) could enable peace with the Chaos Undivided barb faction.
Animals
BTW I've been questioning whether it's appropriate to have generic Animals in this mod, especially as a separate barb faction. I think it could be interesting though to leave them in, have a few wild beasts like Griffon and Hippogriff that could potentially be captured by those with the right skills or magic, and perhaps have Athel Loren and the Beasts of Chaos duke it out for influence over the Animal barbarian faction. A powerful Amber Magic Ritual could also enable peace with the Animal barb faction.
Barbarians
Generic barbarian faction representing all other units effectively under no central control: the basic tribal barbarians in their cities/settlements plus a variety of units occurring via Uprising events, map spawning, lair exploring, etc. I doubt the need for adding any more separate barb factions other than these. In one of my games a Necromancer from an uprising event effectively seized control of a nearby barbarian city, and tormented me with a combination of summoned Skeletons and human troops from his fiefdom. He was hard to defeat; cool gameplay I thought