Well, [url=http://www.belloflostsouls.net/2015/01/40k-deep-thought-chaos-will-never-win.html]I've said it before[/url], despite superficial similarities, Chaos marines are NOTHING like Imperial marines, and should break into a few key types as a result.

1.) Troop Choice - Renegade Marines: like Imperial Marines, only crapper, because they're essentially a warband without access to the Imperial war machine, foraging for guns, bullets and armour. These guys aren't elites; they're space hobos. A Lambourghini isn't worth sh!t if you don't know how to fix it after a crash and can't pay anyone to fix it for you; same principle applies to all Astartes equipment.They should have rules like Heresy-era Blackshields (if not lifted from them outright). Overall, the army should emphasise cheap, disposable Marines, played in ways you would never normally use Marines (tarpitting in squads of 30 or 40, that sort of thing). They should have a wide variety of weapon options but they should all be crap: shotguns and stubbers. All those cheap, crappy weapons lifted from the enemy. These guys should be only one or two steps above common Cultists.

2.) Troop Choice - Chaos Marines: what happens to Renegade Marines when they realise they're never going to survive alone out there without turning to Chaos. They should have very little equipment (for the same reasons as the renegades above), and compensate with improved statlines above regular Imperial Astartes (to reflect Chaos' favour). In gameplay terms, they give up ranged options for improved melee, and should be roughly in-line with regular Astartes in terms of how they're used (teleport options via Warpgate replacing drop-pods, that sort of thing). These guys should still just be Troops though, so nothing really exceptional about them.

3.) Elites Choice - Chosen: Chaos Marines who've risen to the top and have all the best bling as a result. These should be a REALLY expensive option, and work like a melee version of the current Deathwatch Veterans. These should be the only guys with access to things like meltaguns, flamers, etc... (because they steal them from the punier, less Chaos-empowered marines like the bullies they are) but pay through the nose for them. They should also have numerous absurdly powerful melee options which are dirt cheap, as well as a huge selection of Chaos upgrades. They should also be able to mix with Terminators. This should be a hyper-customisable unit that can be tailored to deal with anything, with range options becoming quickly prohibitive, but melee options remaining always the cheapest, most preferred way.

4.) Elites Choice - Champions/Possessed: like Chosen, but with all the range options gone in favour of a brutal statline. They should be in very, very small squads (1-3) and should be a terror unit/hero-killer.

5.) Cult Marines: like Harlequins and Militarum Tempestus, these should have their own mini-codicies, intended for use as Chaos allies.

That's how I'd do it, anyway; each type would basically be the levelled-up version of the type than came before it, reflected in superior statlines, weapons and options, and much, much smaller numbers. There should be a big thing that only the expensive units get Dedicated Transports. Lowly units have the option to either sell their souls and teleport, or else field themselves in huge numbers and eat the bullets.

Chaos should be an absolutely brutal army that plays a horrible attritive style, sacrificing what looks like the whole army at the expense of the champions and chosen stealing all the glory for themselves...

... because that's who they are.