I'd love to see a proper traitor Guard codex, with models for Blood Pact, and all the great tanks/non-guard units that the traitors field in the Gaunt's Ghosts Books.
Like the Stalk Tanks, AT70s, SteG 4 light tanks, Loxatl, Chaos Marine Battle Brother Allies, etc...
I'd love Arbites and Mechanicum from a background perspective as well.