I would rather have Customization widely open for each and every class rather then limits to specific classes. That's just my opinion, since it would cause customization to become more confusing. Although it's a good idea, don't get me wrong on that.
My suggestion was half in jest, but in all seriousness I think the customization should be wide open but with some unique shiny bits that are only available after having reached a certain level of favor for one of the gods -- more skulls for Khorne players than would otherwise be available, some scroll or tome for Tzeentch, etc. Not major differences but just something that makes followers of one god distinguishable at a glance from the others.
Just saying, but you would easily be able to distinguish a Berzerker from a Plague Marine. But yeah, I do agree that classes should be distinguishable from each other.
That is a unique idea. However, if they buff one race they have to apply something similar to the other three. As for your approach I like your initial concept, though I imagine it would be like: Khorne: Increased speed and melee attacks. Nurgle: Increased Health and faster regeneration. Slanessh: Increased player movement and attack speed. Tzeentch: Increased Stamina/Mana for Sorcerers & increased Ranged damage. Though to be fair I think a negative effect would also be ideal for both parties. So for Chaos it would be like: Khorne: Everyone is a target, killing allies. Nurgle: Slow movement and melee attacks. Slanessh: Cannot use ranged weapons, stuck in melee Tzeentch: Stuck in range combat, cannot use melee & slow movement.
The traditional rules are per tabletop. Tzeentch boosts pyskers and sorcerers and adds access to his specific power, Slaanesh increases speed, Khorne improves martial combat at the cost of ranged abilities while defending you from psykers, and Nurgle increases your health and gives you all the saves.
I reckon in battle, Slaanesh followers, as they both get hurt and take damage, they should get more eager to fight. As they take damage and deal damage, they move faster and deal more damage if they're in melee. Hm, sorta hoping you can also be a noise marine and blast the enemy away with dubstep. ... yannow the way I described that sounds a lot like Khorne berserkers. *Shrug.*
I demand cleaving! ALOT OF CLEAVING! Bleeding would also be a nice added effect to my little vroom vroom.
Followers of Khorne definitely need some kind of berserking ability. Whether they get stronger as they become more wounded, or have it as a 20 second buff that makes them deal more melee damage while taking more. They're all about spilling as much blood as possible no matter the cost to themselves. A big thing with Nurgle is always his toughness or his general presence on the field, so it'd make sense to give followers of Nurgle regeneration or debuffing auras. They're tough, slow and great at wearing down a demoralised opposition. Slaanesh is a tougher one in terms of game mechanics. I suppose lifesteal might work with the whole 'pleasure and pain' vibe. Since they can be quite charismatic and flamboyant buffing auras might also make sense, or perhaps they buff their allies when they pull off executions. Slaanesh and his perfect kills yo. Tzeentch? Diversity, man. Should have access to a wide range of mutations, going from a sharpshooter with extra eyes and stabilising tentacles to a three-armed wrecking ball with crab-claw hands. Tactical abilities like invisibility and teleportation would be awesome too. It'd make sense for him to have the most powerful sorcerors though I'd really prefer it if sorcerors were both rare and dangerous. After all, one chaos sorceror can absolutely decimate a squad of infantry alone, and I'd hate to see them included as a straight-from-level-1 class. And like most spellcasters in Warhammer there should always be a risk of a horrendous magical backlash, too. You don't get off lightly if you mess up where the Warp's concerned.