Please remember that these are rumors.
via Faeit 212 from "A must remain anonymous source"
Two types of conversion kits. Unit and Character. Each legion is getting one.
Character conversion kits come with a Legion specific Torso and Head with a sprue of weapons shared between all of them.
Conversion kits consist of 10 shoulder pads, sergeant head, sergeant weapon, and a banner.
All 10 shoulder pads are unique with bits on them, not generic and match various armour pattern mks.
Potentially certain legions will come with a new / unique helmet variant (friend said Ultramarines specifically, as an example).
Cataphract terminator armour is in, and comes with combi-bolter and not a power fist, but an arm ending in a stump, and a weapon sprue with several different power weapon options.
Additional weapon sprues will be purchasable separately, in case you want all swords, all mauls, etc.
No pre-heresy drop pod variant.
couldn't confirm jetbikes or landspeeders, just the infantry upgrades.
Several of the special characters for less notable Legions will not be getting unique models, but the "legion character upgrade" will be based on the named character if you want to create it using those bits.