EXCELLENT potential, decent rule in theory, in practice, rule is
The way i see it is to differentiate recent renegade marines from 10000 years old pissed off legion veterans who should have more superior stats to standard space marine, while of course costing more
Honestly, i am sick and tired of these so called veterans of the Long war getting sweeped by some orks because dice gods decided to be pricks and similar situations, while loyalist space marines, who are approximately few centuries of age will continue to fight, even though they are LOT less battle hardened... I think you see the point, without further ado here is my idea of this rule
HATRED: Armies of the Imperium, +1 LD, automatic regroup and immunity to sweeping advances(ATSKNF basically, but that sounds too noble for these evil badasses ) AS WELL as certain boosts and special rules for certain units, i will get into it in their entries

DITCH THE must issue/accept challenges stuff. If characters kill someone in a challenge, they get to roll 2D6 to see what happens to them
2 Test leadership- if failed, you get a chaos spawn, if passed, nothing happens
3 Champion gets stubborn
4 +1 Attack
5 +1 Strength
6 +1 Toughness
7 nothing
8 Shred
9 +1 Initiative
10 roll 2D6 2 times, re-roll any doubles and any 10s you get
11 Eternal warrior
12 Test LD, if passed you get a daemon prince with power armor, CCW and 1 artefact(if character had any) can be replaced for CCW, as well as wings if model represented had them

Mark of Khorne- Rage and +1 Attack
Nurgle +1 Toughness, and feel no pain (6+),all characters gain access to blight grenades and unit gains access to plague knives(except for terminators and cultists)
Tzeentch, +1 to invurnerable saves, if no invulnerable save, unit gets 5++, unit gains access to inferno bolts
Slaanesh-+1 Initiative, access to sonic weapons on champion

Either drop him to 130 points and keep him as is, or 190 points, and have his sword as AP2 weapon

Drop the points cost to 23 PPM give them frag grenades and allow them to PICK mutations they want, rather than to roll

Lower the base cost to 30 PPM, and if one takes mark of chaos and VotLW upgrade, they become cult terminators, with following changes
Khorne- WS5, +1 Attack, furious charge, 6+ FNP
Nurgle- Reroll toughness tests, and FNP, poisoned shots wound on 6
Tzeentch- 4++, with rerolling invulnerable saves of 1
Slaanesh- Counts as being equipped with Frag grenades when charging through cover,

11 PPM, and keep them as is, Veterans of the Long War upgrade, 4 PPM and gives fearless, and +1 attack and the champion tax (altough 5 pts)
Marks of chaos, keep the price as is, and give them ability to change bolter for chainsword for free or buy extra chainsword for 2 PPM

Lower the cost to 16 PPM, VotLW gives them for 3 PPM 6+ FNP(because they have butchers nails hammered into their skulls for 10.000 years, i think this seems pretty reasonable) and +1Attack
Also, ability to take hidden power weapon or power fist for every 5 models in unit

17 PPM, keep the wargear, rules and options as is, and add the option to take heavy weapons as well and if they take VotLW, for 3 PPM they get to reroll their FNP rolls and toughness tests, also lower the price on champion(5PPM)
Idea is to have them to be tough as hell objective holders and/or nigh impervious to small arms fire like in fluff

15 PPM for keeping the rules as is, if they take VotLW, for 4 PPM unit gets Feel no Pain(because the way i see it, getting shot by plasma/torn up by chainaxes would only excite them more)

Since they are all veterans of the long war, i would include it in their points cost, keep their rules as standard, but exchange slow and purposeful for inability to run(so they can still overwatch)
Lower the points cost to 20 PPM, and give them 4+ Invulnerable save, with ability to re-roll invulnerable saves of 1(like terminators, who i wanted to be rubric termies)

drop the cost to 13 PPM, give them ability to take hidden power weapons or power fists along with special weapon. If they take VotLW for 4 PPM, instead of fear tests, enemy has to take Morale check or fight at WS1(basically fear on steroids, so that only fearless units would be immune to this)

Drop the points cost, 18 PPM, and GIVE THEM THOSE DAMN FRAG GRENADES, and fearless USR

Only thing that prevented CSM codex from being utter , my only problem is that flamer is hull mounted, reverse it back to turret mounted and we're good

Keep the metal box as is(now you will think i am crazy because CSM need delivery method for their CC units. Wait for my idea for entry on LRs it is the task of BIG METAL BOX to deliver loads of troops into CC through heavy firepower )

drop the cost to 19 PPM, if they take VotLW for 3 PPM, they get to reroll dangerous terrain checks and shred on their HoW attacks

Leave them as is