Make all cult troops elites, and make Daemons god-specific, only summonable through a cult troop sergeant/unit (All cult troops count as having personal icons as standard gear)
We should definately have 'cultists'. They are to Chaos SM's, to what Imperial Gaurd are to standard SM's. I do however think they should be Troop-choices straight up. I do see what you were leaning for with your "Word Bearer's" Legion mark, but I personally think cultists should be quite common on a battlefield than rare.
Cultists should be cheap, fragile, weak, but numerous and able to make summoning Daemons easier. (i.e.: when Deep Striking/ Summoned near Cultists, Daemons scatter less - if at all).
Your stats look spot on for the Cultists:
Wargear: Lasgun and laspistol.
May take: An Icon of Chaos Glory (Re-roll failed morale tests) for 10pts. May replace lasguns with close combat weapons at no additional cost.
Maybe even add a Cultist HQ choice. That way we could have proper 'Cultist' Armies. Maintains/improves theme, more model selection, further tactical choices, I can't see how it could be bad. May need change the name of the Codex to "Followers of Chaos" though. Not that that's a big deal right?
For Legion rules:
A Chaos Lord can take a 'legion mark' for a points cost, giving an army boost. Nothing too game-changing.
Black Legion: Units of Chaos Terminators may be taken as troops (To signify all the veterans in the Black Legion), less the 'Deep Strike' rules.
World Eaters: Khorne Berzerkers are counted as troops and you may take Bloodthirsters and Bloodletters as summoned Daemons.
Chaos Lord gains +1 attack and the special rule 'Furious Charge'. Sorcerors (As psykers) and cult troops of other gods may not be taken in a World Eaters army.
Thousand Sons: Rubric Marines are counted as troops and you may take Lords of Change and Pink Horrors as summoned Daemons.
Chaos Lord gains a force weapon and the Psyker rule and may pick two powers from the Chaos Psychic powers list. Thousand Sons sorcerors cannot suffer from Perils of the Warp. Cult troops of other gods and Possessed (As dust is unpossessable) may not be taken in a Thousand Sons army.
Death Guard: Plague Marines are counted as troops and you may take Great Unclean Ones and Plaguebearers as summoned Daemons.
Chaos Lord gains +1 Toughness and the 'Feel No Pain' special rule, and his close combat attacks are poisoned (4+). Cult troops of other gods and Chosen (Being sneaky) may not be taken in a Death Guard army.
Emperor's Children: Noise Marines are counted as troops and you may take Keepers of Secrets and Daemonettes as summoned Daemons. The Chaos Lord gains +2 Initiative and the 'Acute Senses' special Rule. Cult troops of other gods and Obliterators (Being imperfect) may not be taken in an Emperor's Children army.
Iron Warriors: Iron Warriors may take two extra Heavy Support units than is normally allowed. An Iron Warriors army may not include Possessed or any cult troops. The Chaos Lord gains a Servo-Arm (1 Strength 8 attack at Initiative 1) and the ability to repair damaged vehicles of a Weapon Destroyed or Immobilised damage result on a 5+.
Alpha Legion: The Alpha Legion may take Cultists as troops. All infantry units gain the Infiltrate special rule. An Alpha Legion army may not include cult troops or Land Raiders.
Night Lords: Raptors may be taken as troops in a Night Lords army. All units cause a non-cumulative morale decrease as described in the Raptor entry. Night Lords may not take cult troops (Being staunchly Undivided) or Obliterators. The Chaos Lord must take a Jump Pack. All squad leaders have the option to take twin Lightning claws at 20pts for the pair.
I like this. I assume Noise Marines, Death Gaurd, etc... are all Elite choices normally - meaning if your HQ doesn't have a "Legion Mark" your Troops are made up of the Black Legion marines (and/or Cultists if we include them). The above said legions (Death Gaurd, etc...) can still be taken as Elite choices.
Personally I'd go further and have Cultists as the ONLY troop choice, so if you want to field any significant number of CSM you MUST have a HQ with a Legion symbol. I appreciate its a bit restrictive, but it could add for theme.
Regardless I do like this Legion system. I am hopeful it gets adopted as its very similar to what the Space Marine 5th Ed Codex has regarding Chapter Leaders as HQ choices.
I would have some Daemon conflict involved (e.g.: if fielding Death Gaurd you may NOT field Thousand Son's - Nurgle and Tzeentch don't get along) mainly for theme. Alternatively, allow players to be able to field conflicting forces, but bring back in the rules of conflict to discourage players from doing so. It may be childish but sometimes its better to let players feel free to avoid obvious bad decisions.
Then again, it could produce some interesting strategies. Who knows.