Ol' Games Workshop has been doing well in the, "fan service" department. Rumors say we'll get two new armies in December (or shortly thereafter), Imperial Agents and Chaos Traitor Legions.
For the Chaos Codex, I can only assume that we'll see rules like the Space Marine chapters.
The Black Legion will be the generic Ultramarines style army that's flexible enough that most current Chaos armies can play. Death Guard, Emperor's Children, World Eaters, and Thousand Sons will probably have some of the best rules/tactics/formations because they already have rules and units based on the single Chaos god they are devoted to.
The thing I think I like the most, GW really seems to be putting the things out that fans want.