These are rumors so please take with the standard salt. This compilation of rumors is summarized and put together by the Dude, with some of the commentary taken out of them.
theDarkGeneral has said the book should allow players to field Legion-specific armies that have a uniqueness similar to older Dex's whilst simultaneously allowing fexible mixed force builds.
theDarkGeneral has clarified that HQ selections may change what units "count as" (unclear if this means FOC slot used or counting as scoring etc), wargear upgrades for squads and vehicles, access to Daemons, and possibly affect unit sizes.
theDarkGeneral has claimed that there could be many more flying options in this book, including "Chosen with Jump Packs (expensive), Possessed with Wings (kinda expensive), Chaos Assault Marines (average), and Raptors". He also implied Cult troops could possibly get access to Jump Packs changing their FOC slot, but that part of his post could easily have been wishful thinking, especially given he also said there could be some tough choices, with most units unable to move FOC slots. He did mention he had heard it may be possible for certain units under certain conditions to occupy 2 different FOC slots simultaneously, but was dubious about this.
He has gone on to say that Chosen and Possessed could be recieving the most significant overhaul in the book. There is a chance HQ characters could take a unit of Chosen as a bodyguard as an upgrade, thereby not taking up an Elites slot.
Possessed will be more viable, losing the "game costing randomness" curretly on offer. theDarkGeneral claims they could become like the Ogre Kingdoms Maneaters, picking abilities from a list before the game. theDarkGeneral has also suggested a character may allow one or more units of Possessed to be taken as Troops.
According to theDarkGeneral, Berserkers could see their Rage rule return to the old Ld test system, where a failure results in Rage as we know it. He also said that 2 ideas for Chain Axes have been floated. One being simply giving them Rending, the other being +1 Strength. It is unclear which, if any of these ideas will make the cut.
theDarkGeneral has implied Alpha Legion may be able to pass on their Infiltration abilities to their transports.
theDarkGeneral claims Lash of Submission may get downgraded in line with the Daemons Codex version and that a suite of vehicle-mounted sonic weapons were at some point play-tested for Dreads and Preadators, adding there could also possibly be a new heavier sonic weapon with some extra tank-busting capability.
theDarkGeneral has also suggested there may be one or more large Apocalypse sized released for Chaos in the near future too, possibly including some kind of Autocannon-toting flyer, although what this may be is unlcear.
There was also some suggestion from theDarkGeneral that the Havoc Launcher may be upgraded to two shots.