They've been dribbling out rules hints, and there've been a couple of big aligning-with-official-hints leaks, and I'm coming around a bit (World Eaters legion rule notwithstanding

). All told, it looks like we're looking at:
List building from Space Marine v1
Core unit statlines from Space Marine v2
Core rules for orders / shooting / combat from Space Marine v2
War engines with hitpoints from Epic Armageddon (although on the surface they look slightly weird - leaks say a questoris knight has 3 damage while a warlord has 6...)
Aircraft rules from Epic Armageddon
Concepts of USRs, unit types, and weapon traits from Epic 40K & Epic Armageddon
Also we've got leaks of legion rules and they make me think I'll definitely go with Ultramarines, as theirs is a nice simple shooting buff for targets that have already been shot at by a formation from the same detachment, while the Salamanders (who I was also briefly thinking about as another loyalist legion I haven't collected before that actually has colors) get morale boosts, which are generally dependable but boring.