A harlequin one would be cool, as id love to see plastic harlequins and things like the death jester would make a great plastic character clampack.
Heres My Picks
Brood Nest (To be re-released in limited expensive resin)
Arbites/Custodes
Chaos Legions
Digganobz
It would be best to ignore the unit composition rules from the Eldar/Dark Eldar books. That represents a small group of harlequins visiting a Craftworld/Kabal. A supplement would be to field Harlequins as a stand alone army. They would be organized differently as a stand alone force.
Also, the old rules for Harlequins allowed Wraithlords as heavy support choices. I always liked to think of them as solitaires that the Laughing God was able to save from Slaanesh
I'd love to see more Imperial Guard regiments.
Also, mini codices on the Adepta Sororitas Orders.
Being a hardcore eldar player, top 2 codex/supplements I want : Harlequins, I've got a whole bunch of the old rogue trader harlies + some current ones, would be nice to have a new playable army, I've got the models waiting for it. Exodites, according to info from FW, they are working on this one, they will be the ones releasing it.
Already got corsairs, with an update coming up, a very fair codex and some cool minis, now I need some exodite minis!
Obvioiusly the missing craftworld supplements would be nice to have.
I'm also a dark kin player, something like a Black Heart Kabal supplement would be welcomed with much joy from my part.
Being a Nid and Tau player, I'm rooting for a Genestealer supplement and A kroot codex comeback, no rush though as I still got too much other stuff to work, but it would be nice!
https://www.facebook.com/DarkMoonKabal
Id like to see a Codex; Chaos Cults list too. Possibly designed to ally with Alpha Legion..? Some odd mix of chaos cultists and traitor guard? That'd be cool.
Rak'gol would make for a great mini-dex.
Genestealer cult would be nice but I would like to see any tyranid supplement codex we had hive fleet lethiavan in electronic form so something like that in hardback or even a deferent one maybe something fir the big bugs would be cool.