Based on Gen.Steiner I would like to see:

-Codex: AdMech/Dark Adeptus
-Codex: Exodites
-Codex: Defenders of the Imperium (PDF, Arbites, Ecclesiarchy)
-Codex: Rogue Traders (Squats, Mercenaries, Rogue Traders, etc)

Those would bring ore diversity to the existent 40k universe.And no new races until all the codexes are on the line.