So, in the interests of baseless, rampant, scurrilous and perhaps even heretical speculation, which army/force/faction from the 40K universe would you like to see brought into the game? Who deserves a full codex and a broad range of miniatures in your opinion?
Given the origins of the game, I do feel that it's a shame we don't have any Rogue Trader options. Brave (or greedy) explorers traversing the galaxy with their crew offers a lot of scope for customisation and conversions. From the captain and his retinue, to the armsmen of the crew, you could have plenty of options for modelling. You wouldn't necessarily be stuck with the same old Imperial options either as you could add in more esoteric or exotic designs that the trader has picked up on their journeys.
Failing that I'd suggest Generic Marine Chapter 17-B as being worthy of it's own codex.
Seriously though, what would you like to see brought to the game?