Squats or (Genestealer) Cults would be cool but, more realistically, I'd settle for Dark Eldar...
Codex Adeptus Mechanicus
Codex Eldar Exodites
Codex Adeptus Arbites
Codex Chaos Knights
Codex Mercenaries (Demuirge, Hrud, Kroot, units, etc...)
Squats or (Genestealer) Cults would be cool but, more realistically, I'd settle for Dark Eldar...
More aliens! Preferably mercs so we can get some different aliens. There are currently 9 full human codexes out of a total 16 armies, and that is shoving all the variously flavoured space marine chapters that share the rulebook into one. I am sick of humans. There are more races than humans and I would like to see more focus on them.
Kabal of Venomed Dreams
For me it's a tie between Ad Mech and an all Kroot codex. Because Ad Mech already has so many votes, I voted mercs so they get some equal love.
I built an Arbites army off the IG codex and then I found a fandex on dakkadakka. It would be really nice if GW would go back to the older lines and include them (arbites,exodites).
Adeptus Mechanicus would be my first choice. Though Exodites comes in very close just because I want to have an army that'll let me sing the Dino Riders theme song. Third place goes to the Mercenary one, though it too almost ties with the others for most desired.
The other options could easily be done as a dataslate that modifies a parent codex slightly. Or perhaps just a digital codex ala Inquisition and Legion of The Damned.
I voted Arbites but what I really want is a proper Black Templars codex. Vows, expanded unit choices, maybe a new special character. Oh, and proper fluff. Not this "we worship the Emperor and are cool with Psykers now" stuff that got put into the Marine Dex.
Mercenearies, high time we had the ability to have mercenaries, and once more allow our old illegal models to have a good dusting off again, and the old Necromunda gangers too.
Astra Miliwotsit? You're in the Guard now son....
Tyranids!
orkz.... waaaggghhh