Super excited about the numerous possibilities with the release of Supplemental codexes. So far we've got ones for Eldar, Tau and Chaos Marines. Here's some I would love to see...
Lost and the Damned - Rules for Chaos Beastmen, a more dedicated Traitor Guard, psycho heretics, big mutants...basically everything that was in the old Codex: Eye of Terror list.
Death Guard - Maybe add in a new breed of zombie trooper?
World Eaters - Anything to help these guys reach combat more intact would help. Maybe some sort of crazed gladiatorial type troop?
Death Skull Orks - Lootas galore!
Genestealer Cult - Patriarchs, Hybrids and Limos. Creepy!
What sort of lists would you like to see?