Ok, so, as you may know, I've put up some Chaos Homebrew in the Homebrew Rules, and am working on a pile more.
To be concise, I am writing 2 concurrent Fandexes : Chaos Legions and Chaos Renegades. Legions will cover World Eaters, Thousand Sons, Emperor's Children, Death Guard, Night Lords, Black Legion and presumably Iron Warriors (unconfirmed atm.)
Renegades will cover Red Corsairs, Fallen, non-Chaos Renegade Marines (except Knights of Blood, obvs) , Traitor Guard, Mutants, Cultists, Alpha Legion and Word Bearers.
In a very loose way, Legions is based on the EoT, the other not really.
Now, obviously the Big Five Legions are sorted, and need minimal work apart from points adjustment (WE, EC, DG, TS, BL) and minor issues I already consider sorted tbh. (In MY rules, not IRL)
The Word Bearers, Alpha Legion, Night Lords and Iron Warriors are more tricky.
Here are my provisional ideas on them, feedback gratefully received.
With NL and IW in the Codex with the Cult Troopers, and the Cults likely being Troops, they need to be comprised of a separate Troop unit called 'Traitor Legionnaires'. They need an unlockable bonus, working like the Chapter Tactics mechanic - that is, when you take a Character relating to that Legion (ie, a 'Warsmith' for IW or a 'Reaver' for NL) the rule they have by default (necessarily bland and vague, so as to promote Legion-themed play) 'Ancient Enemy' or 'Eternal Hatred' or somesuch, will be replaced/overwritten by:
NL : - Raptors become Troops. All models with 'Ancient Enemy' gain the Hit and Run, Fearless and Night Vision USRs. [This needs work, it feels very underpowered
IW : - All models with the 'Ancient Enemy' rule gain Melta Bombs at no additional cost. In addition, they gain the Relentless and Stealth USRs. [See what I mean? I also feel there should be a way of moving Oblits to Elite or HS a la MotF.]
With the AL and WB this is more complicated.
The Codex they reside in will have alternative Troops, in Traitor Guard (unlockable, starting as Elite) , Beastmen, Renegade Marines, Cultists...maybe Mutants go here too.
Traitor Legionnaires will be Elite, along with Cult Troops (possibly even omitted entirely, meaning you would need Team events/Apoc to field the full gamut of mortal servants of the Ruinous Powers.)
Traitor Legionnaires would move to Troops in the conventional FoC shifting way, with an HQ. While I would do this with Cypher too, I have no intent of letting the cat out of the bag regarding my thoughts there.
'Dark Apostles' would move Traitor Legionnaires, and give them the following:
Models with the 'Eternal Hatred' rule replace that with the Fearless and Preferred Enemy USRs.
The 'Cult of the Hydra' would move Traitor Legionnaires to Troops, and replace the 'Eternal Hatred' rule with the Scouts, Infiltrate and Hit and Run USRs. [Tempted to add Stealth. Too much? I like Alpha Legion.
To alleviate fears - no, I won't be omitting Daemons. They will function as now, but you will be able to purchase Marks for them, with the following effects:
Khorne - Power Weapons, Furious Charge.
Slaanesh - Rending, Fleet.
Nurgle - FnP, SnP, Poison (4+)
Tzeentch - 3++, access to a psychic shooting attack.