We have met the enemy and he is primarus

So these guys are coming.  Upgraded Space Marines, bigger, better, with better gear. From a background standpoint, this is a huge can of worms being opened.  Some of the worms I can see resolved more easily than others. Developed in secret for 10,000 years?  I can maybe see that with the AdMech.  Perhaps Guilliman made Cawl program himself to not reveal the project until Guilliman