Codex Supplements are not the priority releases for Warhammer 40k, nor will they ever push around the codex release schedule. Instead they have been said to be a labor of love within the design teams. What the game designers would love to see, and want to work on, then are worked around into the release schedules.
So what is on the designers dream list? Here is a look at what just might be floating around in the near future for codex supplements.
Something to think about: There are no supplement releases without an updated codex.... hint hint.
Please remember that these are rumors.
via Stickmonkey on Faeit 212
I saw the supplement post, I wanted to add to it some I've heard being talked about, no timelines, no state of completion, just what I've heard going up on the designers wish list, sorry:
(Rounding out the big 4)
Kraken,(interestingly no mention of Behemoth)
(And mention of Catachan Devil has come up a few times)
Like I said this is the designers wishlisting i've heard, may have missed some, but a little more food for thought while your on the topic...