So I've been thinking a lot about Chaos Demons, and how they might be implemented. This also led me to an idea about and update I think they could do for Chaos that would add a lot of flavor and variety to them. Obviously, this would fall under TW:WH3 ideas, but it's definitely something I think should be put out there.
I think one of the things that's really missing from the Chaos armies is a way to show the gods favor/devotion to the gods. This led me to an idea that would work sort of similar to the new VC bloodlines.
Marks of Chaos.
There would be four Marks of Chaos available for a chaos army (could be lord-specific or campaign specific), one for each god (Khorne, Nurgle, Slaanesh, Tzeentch) and each mark would have 4 ranks to it. Over the course of a campaign, you'd unlock four points to be spent upgrading the marks. Each mark would grant bonuses to your armies (both campaign and battle), thematically tied to each god.
For example, Khorne would increase your weapon strength and damage, post-battle casualties, have a chance to cause all enemy units to enter frenzy and low healths. Nurgle might cause your lords to trigger attrition for all enemy armies in whatever region/province they're in, cause nurglings to spawn from dead enemies in battle. Slaanesh might reduce public order (as people go crazy to hedonism) in the province they're in, make your units immune to moral loss. Tzeentch would increase your spellcasting by granting some unique spells, increase enemy miscast chance, and mess with winds of magic in your armies province.
Each army would also be granted a spell to summon a unit of demons during a battle, and what demons it summons would depend on which Marks you've put points into. If you get the first level of the Mark of Nurgle it'll summon some Nurglings, second level Plaguebearers, third level, Beast of Nurgle, fourth a Greater Unclean one. Other marks would grant equivalent demons, but you can only summon for the mark/level you've put points into. If you put all of your points into one mark then you can only summon demons of that god. But if you put, say 2 points into two marks you could summon the first two tiers of demons from either god. Could do 3 and 1 points, or 1 into all of them, granting you some of the bonuses from every god.
Putting points into the first tier of each would trigger a new trait, Chaos Undivided. This would give you some additional bonuses to make up for missing out on the higher tiers of the four marks, and also allow your summon demon spell to summon a random demon from any of the four gods. So if you try and summon a greater demon, you don't know if you'll be getting (Lord of Change, Keeper of Secrets, Bloodthirster, Unclean One), but that also means you have a wider array of demons available.
I think the Chaos Demons shouldn't operate in the same way as most armies (build up cities), but I also think they should be more distinct from the Chaos armies (warbands), so I had an idea that might sort of meet in the middle and play into the theme of demons pouring from the warp. The demons would have a Hell rift (or some sort of base) in the far north of the chaos wastes (up near where Archaeon starts?), and this would more or less serve as their capital which they would use to upgrade buildings and train lords and heroes. However, when they conquer a city instead of just turning it into ruins they would actually have an option to open a warp rift in province capitals only, and only once the entire region has been conquered (turned into ruins).
This warp rift would allow them to teleport to any other rift they have open, allowing them a lot of mobility around the map and the ability to travel back to the warp. The cost of this could be that they can only recruit more units when they are near the hell rift in the chaos wastes. You could even block global recruitment, forcing them to have to be near a warp rift/the original hell rift to really offset for the added mobility. This allows them to be mobile like chaos currently is, and still have losing an army/suffering high casualties in a battle to be more devastating than it would be more most other armies while also being thematically appropriate.
To prevent them from rushing to a province capital and building a rift, or allowing another army to take over a town while a rift is present, you will be blocked from colonizing ruins in an area where the province capital houses a warp rift, forcing your enemies to go after your warp rift (which would work similar to a mix between Wood Elf outposts and the High Elf gate cities). You'd be able to upgrade the garrison similar to the High Elf gates, so they aren't just a bunch of basic units like the Wood Elf outposts in the late game (indicating the ability for more demons to come through the rift to defend it), and obviously they'd cause corrupt to the entire province in addition to blocking the minor settlements from being colonized.
Anyways, these two ideas have been bouncing around my head for the last couple weeks so I figured I'd put it out there to CA and the community to consider.