Please remember that these are rumors, and to take with a grain of salt. We are still a ways out on this codex.
Via Tastyfish who compiled all the information into one place
rumor sources include Stickmonkey, ghost 21 and others...
New Heavy support platform. Basically immobile deep striked! heavy weapon and crew. Rail and Ion Cannon, and missile launcher are all obvious options here, but nothing solid with the rumor.
Assault Skimmer. Something between size of piranha and hammerhead, fast skimmer open topped, assault on disembark.
Update: Yes, its assault after a flat out move, and has the "drop troops along path" option similar to the storm raven. its supposedly a dedicated transport option for one new unit. (I dont place faith in this last bit, its very rare in current GW systems to see a vehicle kit limited to one unit/role...if GW is trying to sell them they will be available to more than one unit, or be a separate entry unto itself.)
New Drop ship. Not the FW one, but something new. Holds a boatload of fire warriors, or a few battle suits.
New battle suits. Not sure if these are the "next gen" suits or just heavy suits redone with out metal bits.
All current metal sets to be plastic (Pathfinders, Krootox, Vespids). New plastics are completed for Crisis Suits, Stingwings and Pathfinders.
Plastic Barracuda as well as a new unit choice, which will be a new alien race (insert your fantasy here).
Battle Suits - plug and play system. Essentially streamlined. Pick two guns and a skill or two skills and a gun. New models with interchangeable arms similar to Killa Kans?
Rule Changes
HQ and Troops
HQ choices are used to unlock units for troops, so a commander in a Crisis Suit unlocks Crisis Suits as troops, an Etheral unlocks Fire Warriors, Shaper unlocks Kroot.
Crisis Suits
Mostly same stats but now 3 wounds, come with every gun option and you can choose what weapon systems you fire every round. Max squad size x 5
Broadsides
Can take Network Markerlights instead of the missiles on their arms
Hammerhead Railgun
Draws a line across the table hitting everything under it. Multiple penetration through vehicles, only stopped by a glancing hit.
Shield Generator
Only team leaders can take a shield generator. The shield generator functions exactly like the DE Shadowfield. 2+ invulnerable save until the first failed save at which point it ceases to work.
Drones
No longer bought as Wargear. They are bought as a unit and then divided as you see fit at the start of the match. You can mix your drones when bought for the point cost needed. This caters for Drone Squadrons as an option.
Markerlight
One markerlight can fire the seekers off one model. Seekers replenish and are no longer one use items. Skyrays will only be able to fire one seeker per round.
1. Kroot HQ is in the works!
2. Kroot to get own transport (Devilfish will no longer be able to transport Kroot)
3. Kroot will get light armor saves (6+)
4. Kroot shaper will allow for the unit to have a special genetic trait.
5. Firewarrior carbine to get a grenade launcher (either explosive or EMP)
6. Vespids reworked and getting a Heavy unit
7. Firewarrior BS increased to 4
8.Crisis suits will get “hit and run” ability
9. Various changes to Firewarrior equipment
10. Various changes to Crisis suit equipment
11. New Prototypes
12. 4th edition prototypes now “standard” equipment for all suits
13. All HQ’s can act as a commander
14. New type of “Heavy” Battlesuit (new prototype of the Broadside)
15. Stealth suits will be changed to a advanced scout unit similar to Pathfinders
16. Pathfinders will be changed to act as a type of Tau Commando infantry
General Codex rumours
Not the next book, but probably after Black Templars
Author may be Phil Kelly
Army has 4 basic HQs and 3 basic troop choices excluding changes made by any Special Characters if there are any
Return of 4 old SC, plus 2-4 new ones
No Court of the Archon/henchmen type mixed units
New Tau models
New SC "Iceheart"
Crisis suits have been completely redesigned (not just recut sprues and ankle tweaks), less boxy and including a 'morphic weapon'
New sniper suit
Shas'O get a stealth armour option, may be an experimental suit like Shadowsuns, or the experimental suit might be a third option
Allies and Auxiliaries
Vespids get two slots (unknown if this is an additional two, or two overall). Kroot get an additional unit
No Knarloc riders,though perhaps something big (new WFB monster-szied) for the Kroot
Maybe minor psychic powers for Shapers or Shaper like character this is something Ghost has heard from other people and not stated directly
Demiurg have 1 HQ, 1 troop and maybe 1 else. Presumably the Elite choice mentioned elsewhere
Demiurg ancients are a thing Probably HQ, but could also be the elite unit
Possible Demiurg elite choice
Demiurg are "equal to banshees without the scream" so presumably anti-armour close combat, or at least have that as an option. This might also be elite unit rather than the troop one, or be a unit that does both - like a kit that would let you make Boyz and Ard Boyz (or Dwarfs and Longbeards). An elite unit, but being an upgrade to a troop slot to keep out the way of the battlesuits
Demiurg and one other new race join the army, the latter being something bizarre or at least something we've not seen before.
No Gue'vesa unit or models
No Nicassar
4 box sets, and 4 blisters in wave 1
Demiurg plastic box
Battlesuit plastic box
Demiurg ancient blister
New Tau SC blister