Some of this might be a re-post, but most of it is new (from Faeit212):
This comes from a very solid source that has helped us verify rumors in the past
via an anonymous and solid source on Faeit 212
World Eaters, Thousand Sons, Death Guard and Emperor's Children are getting supplements eventually.
The elite unit entries in the core codex don't necessarily reflect World Eaters, just Khorne Berserkers, for example.
The Legion lists will have unit entries for Berserker/Plague/Rubric/Noise Terminators, unique warlord traits, and some fun wargear stuff.
Tau have two more books; one for kroot and one for "the rest" of the expanded empire.
The above rumor was mostly a sources a response to the rumors posted that there would be no mono-god legions, and that Tau would be receiving more than just a mercenaries supplement.
via an anonymous source on Faeit 212
Not all sub factions within 40k will receive supplements. Only those who have the potential to bring an alternate yet characterful play style to the tabletop and have the scope for expanding the lore.
Just over half the space marine chapters of the first founding will receive a supplement.
The mono-god legions will NOT be getting a supplement. It's felt they are well enough reflected in the codex. Iron Warriors, Night Lords, Word Bearers were given as Examples of Chaos Legion Supplements
Out of the four remaining craftworlds, there's only plans to turn three of them into supplements.
Tau are not scheduled to get a single additional supplement, but instead there's going to be three!!!
Tau Mercenaries Supplement Verified
via an Anonymous Source on Faeit 212
another Tau Supplement is in the works, it lines up with a few other rumours I've heard about the Tau before it was released to the world. Vespid and Kroot HQ choices, and the ability to play a non "Tau" Lists.
via Stickmonkey on Faeit 212
I have a little more on the Tau mercs supplement.
First, I was told this wasnt planned be out til Q1.
Second, I was told that either this or one of the eldar supplements would be the first with a model wave to go along with it. I wasnt told which models or which eldar supplement.
Codex Supplements via Stickmonkey on Faeit 212
I'd heard Biel-Tan was being worked on after Iyaden, and Saim-Hann was later. But from what I've heard lately it seems the supplements are pure labors of love, and not necessarily slotting into releases like we've traditionally thought. That they are following a "when it's done" release scheme, and not strict deadlines. Multiple digital supplements in a month would also not be unheard of.
There is a lot going on. I've heard scattered rumors of 2014 seeing many new models introduce to codexes with rules in box, but I haven't confirmed this.
via Stickmonkey on Faeit 212
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)