With Codex Imperial Knights and Militarum Tempestus, Gw has finally dived into the whole "mini codex" idea. Which ones would you like to see? It seems these are primarily designed to be played as allies to existing armies, but can be used on their own in a fairly limited capacity.

I am outright going to say : Sslyth.

1) non-humanoid (which is really f'ing needed ffs)
2) fills the "lizardmen in space" niche (but isnt a straight boring port of the WHF version)
3) Adds another playable Xeno race.
4) Can ally with at least Dark Eldar (and perhaps a few others too)

They could probably release an infantry boxset that makes 3-5 models with it being a duel kit of course, plus a Sslyth specific vehicle kit (also a duel kit).

i'm sure there will be the usual splurge of squats/demiurg, genestealer cults, chaos guardsmen and exodite eldar, but would love to hear more original suggestions if you got em'