Dawn of War Codex

Chaos is the sole race with the most AoE...

i allways thought eldar has the most aoe followed by ig

Chaos don't need another AOE attack. In fact, half of the AOE Chaos Globals were nerfed in ELITE. (Dark Flames, LTGB...)

A Mark of Nurgle for CSM or a subcommander are the only things I would add to ELITE. MOT CSM aren't very cost effective against Infantry armour IMHO.

Or simply remove (or reduce) the high melee damage of the CSM AC and give to the squad something more useful for his role.

I would love to see a Nurgle Dread. Kind of an improved version of a standard dread. An improved autocannon (more damage? increased splash radius? aoe effects?) and a bit of health regeneration, of course as a compensation the speed should be reduced. And an ability maybe?

If a specific role is needed for the marks, I'd suggest these:

CSM
Mark of Nurgle: the bolters envenom the objective, it would be good for dealing constant damage against non-heavy infantry, like guards, eldars and most tiranids. Also there would be the increased health.
Mark of Slaanesh: they would have increased melee attack and charge speed they would be good defending themselves in melee, and could use the cacophony for a limited protection against heavy melee attackers.

They would add to
Mark of Khorne: effective against anything in melee
Mark of Tzeench: effectinge against heavy infantry and vehicles.

For me, Mark of Nurgle CSM should be what some of us are suggesting for Plague Marines in a while, the "firm grip" version.

Essentially make them have the bolter in one hand and knife in the other, making them fairly good in ranged and melee.
Not as good as KCSM in melee and not as good in ranged combat as TCMS.

You can then add all the beautiful things like venom, more health, less speed as you like.

My Havoc version i already wrote about it.

I don't think Slaanesh mark should be ever implemented... and I'm the biggest Slaanesh fan here.