Why have the Ecstasy ecstasy rule? Is there a weakness in the list this is supposed to address? It seems superfluous.
I think you have more infantry types than needed. Non-Fearless legionaries are definitely needed so the Decadent makes sense. I think you can tweak the others down.
- The Possessed are basically 2 entirely different units crammed together on one stat line and one formation. If you don't drop one they at least need to be broken out separately.
- The Scout Possessed aren't needed. They have the same stats as Decadents, just adding Scout and First Strike. It's very "like these guys... but better."
- I don't think Havoks are needed. There are plenty of fire support options in the list, including iconic Slaaneshi options like the Knights and Questor.
There are also modeling issues with having too much variety in infantry. There are a limited number of chaos marine infantry models readily available.
I don't understand the mixed formations. I can grasp the desire to have non-Fearless units mixed in with Fearless units so the formation has pressure act "normal." I just don't know whether that's really necessary. An all-Fearless Noise Marine formation is only 6 units. That's not nearly the problem that larger Fearless formations present.
Mixed formations force the players to have to count up a greater variety of weapons and attacks every single action. You get into detailed management of unit placement because you have to manage unit ranges and suppression and so on. Basically, it adds fiddliness. Because of that it should increase the game quality so that it's worth it. I have a hard time imagining that is true at the moment.
The need to have easily distinguishable models is also a bit annoying. Having things both match and be readily identified is rather restrictive in terms of modeling and painting. It makes using older models and any sort of "counts as" nearly impossible. (This goes hand in hand with issues above about variety of required models, obviously.)
I'd also drop the Elite/Auxiliary split on the list structure. There are only two reasons to build in restrictions to army list structure - flavor and balance. I don't think either applies here. The Black Legion list allows fairly extreme builds and (excepting the massed-WE concept) they have been balanced, so there's no reason to think the EC will cause major balance problems. As far as flavor, none of the support-heavy focused lists seem antithetical to the legion's structure or the list concept. If someone wants an army with 4 or 5 bike formations or an EC armor wing, I don't see anything wrong with that. Core/support should be sufficient.
