Which Chpater of Space Marines/ Traitor Leagion/Xeno species do you feel most deserves a codex that does not already have one? Possibly one of the First Founding Chapters, or perhaps a rather newer Legion of Chaos Marines organized post Heresy or maybe a certain Eldar Craftworld?
Personally I would like to see the Iron Hands get a codex. I mean they didn't even get a spec ial charter in the Space Marine codex and thier the only first found loyalist chapter that didn't. I feel they are different enough from the standard marines to write a codex for. I think they would use alot of dreads and tanks and have bionics everywhere but on average be slower or more expensive then your standard Blue Boy. I also don't see them using psychic but instead have them allowed Tech Priests of somesort. The inclusion of a special character with a phase bladde (li.e. the living metal weapon like necrons have) would also be interesting.
I would also like to see the Alpha Legion get a codex. Just like the Iron Hands they don't have any special characters or anything and they are a first founding chapter after also. I mean they could really have some fun with these guys. There whole deal is setting up small cabals of worshipers deep inside Imperial planets and also sneaking into loyalist units and at the right moment turning on them. The mere idea of some of my own men turning on my units mid game is crazy. I honestly don't know how GW would balance this though, maybe have them deepstirke into units or somthing.
I would also like to see the Sons of Malice get a codex but I know it will never happen as they are the Chaos Space Marines that worship the Chaos god Malal. Malal is the god of chaos fighting chaos and so the Sons of Malice would be like good chaos marines... an odd concept I know.
As it pertains to Xeno, I don't know. I think thge Hrud would be cool but really hard to nail down as their race is not unified and they all are natural teleporters.