GW is totally ignoring the 8000lb gorilla in the room. After all, this isn't a mere "rules update" or "adjustment", this is supposed to be a whole new edition! If serious changes need to be made, now is the time to make them!
"Oh, but it must stay balanced and the 3 vs 4 with a d6 is well done." Not really, it is a fairly simple layout and could be done with any other die after adjusting. There are lots of good reasons for giving some more flex to the stat system, but GW would have to do a recalculation of the odds for conversion into a d"X" system which would allow for wider stat margins. That part would be easy, but the hard part would be rebalancing the weapon prices and stats. An easier way in many cases that would leave the dice alone would be to give Marines another effect for their genetics; either some version of FNP (say 5+) or an extra wound. It really would be fairly cheap to add (+3-5 points per model). It would also make Battle Sisters something more than IG vets in power armor with bolters.