Though there's talk of having multiple Chaos codices, I don't see a real need for it. There isn't a need for so many SM codices, due to the whole Chapter Tactics system. You could consolidate all the Marine codices into one book, other than Grey Knights. Even Space Wolves could be fit in without losing their uniqueness, as most of the difference in their army comes from a handful of unique units (Wolf Guard, Thunderwolf Cavalry), and most of the other difference could be changed simply by giving them a unique Chapter Tactics.
So they should have a system where you pick your Chapter (or the Founding Chapter your chapter came from), giving your dudes a unique Chapter Tactics and unlocking a couple of unique units and some Force Org swapping. Dark Angels would get armywide Stubborn, and Bikes and Terminators as troops.
Same principle could be applied to the Chaos Legions. Pick your Legion, get a unique set of special rules, and unlock certain units and FOC swaps. Picking World Eaters gives normal dude Furious Charge, moves Berzerkers to Troops, and unlocks Bloodletters and Juggernauts of Khorne. Night Lords, on the other hand, wouldn't get Bloodletters, but would have access to Raptors and Bikes as troops and have Hit and Run/Acute Senses on everything.