Let us start with 6th edition fluff progression.
The Imperium is fracturing and the Space Marines are starting to separate themselves from the Lords of Terra. The heretical and xenophobia has gotten to a tipping point causing many chapters to take actions against the “best wishes” of many in the Imperium.
A discovery of galactic importance has happened.
At a time before right before the Horus Heresy the Emperor had intrusted Roboute Guilliman in the protection of one Xenos race that was completely immune the temptations of Chaos and would prove the ultimate key to the destruction of Chaos. The Horus Heresy ended such plans. Fast forward today with discovery of these lost correspondences, it is believed by the Ultramarines and others that the Tau are that lost race. So now instead of being charge with their destruction the Astartes are the Tau protectors.
Every unit gets a 6+ save vs. all Psychic Powers.
Random Charge Lengths are in
Pre-measuring is in.
Random Battle field effects are in.
All armies can purchase buildings for placement on the battlefield.
Psychic powers are selected during deployment. (except Grey Knights)
Deployment and Missions types have doubled.
Get ready for the Big one..
ALLIES ARE BACK IN
That is right allies are back in and this will be according to fluff and will have certain restrictions. So for instance Tyranids will not have any allies. Tau with all Space Marine Chapters. Necrons with Blood Angels. Imperial Guard with Space Marines. Chaos Space Marines with Demons. If it fits the fluff it will be done.
This is just to get you guys started.
Expect more to come out as we get closer to the impeding release of Warhammer 40k 6th edition.
Oh again forgot to mention in the CSM rumor post to add that Oblit options will be even more expansive with close combat load out available.
LOL PROTEGER TAU QUE MOCA