I mentioned this in the "If you owned GW" lore thread, but I had an idea for making the whole Primaris/Cawl/Guilliman debacle a bit more interesting than "Shiny new Astartes fix everything". TLDR: Imperial Civil War (which has been suggested before I believe and even hinted at in official literature) but I've got some specific ideas for it.
The basic idea:
Guilliman, Cawl and co are not quite as popular as they'd like to think. Cawl's meddling with the holy Astartes geneseed template has outraged certain factions within the Inquisition, the Adeptus Mechanicus and the Space Marines, and Roboute Guilliman's resurrection has been viewed with great suspicion by many, some believing it to be the work of Xenos witchcraft. For their part, Guilliman believes the Imperium has become backwards and corrupt, and requires a complete reformation if it is to survive into the future, whilst Cawl is firmly of the opinion that the current state of Imperial technology and science is unacceptable and must be advanced- along with humanity itself if need be. This conflict between the "Traditionalists" and "Reformists" eventually escalates to a schism in doctrine and dogma, and tragically to open warfare. Cue Imperium Secundus 2: Partisan Boogaloo.
More advanced (and less organized) ideas:
One of the main sticking points that starts this schism is of course the presence of the Primaris. The Traditionalists believe that the Primaris are a necessary evil at best, and outright heresy at worst. The Reformists believe that they are crucial to the survival of the Imperium, with opinions ranging from "They are our greatest asset against the encroaching darkness" to "Firstborn are obsolete and have no place in the modern Imperium". Not all Primaris actually agree with this outlook, it has to be noted, and many Primaris join the Traditionalists. By the time actual civil war kicks off of course, the Reformist Astartes side is made almost entirely of Primaris, with the only non-Primaris being those that cannot safely cross the Rubicon but still pledge allegiance to Guilliman and Cawl.
Of course it's not just "Totally not True Scale Marines we promise" that are causing trouble. The state of Imperial technology and science is in Cawl's firing line; he perceives the current stagnation of innovation as utterly counterproductive and driven by pointless nostalgia. As such, a schism within the Mechanicus begins, between those that wish to see the Imperium reclaim technological superiority, and those that believe that existing technology is sacred and must be protected from blasphemous revisionism.
The Ecclesiarchy isn't much better off either. Some are hailing Roboute as a new living saint who has brought order back to a fractured Imperium. Others see him as the puppet of vile Aeldari witchcraft, calling him "The Revenant King" and believing that he risks bringing the Imperium to its knees. Existing theological divides widen, and even the Sororitas find their separate Orders coming to blows over whether he is a herald of salvation or doom.
On the Reformist side, you obviously have Guilliman, Cawl and friends. Astartes chapters on side include the Ultramarines (of course), Salamanders, Iron Hands and Imperial Fists amongst others. On the Traditionalist side, Gabriel Seth, Logan Grimnar and Inquisitor Fyodor Karamazov- the main Astartes chapters are the Blood Angels, the Dark Angels, the Space Wolves and the Grey Knights.
...And that's about as far as I've got with this idea so far. There will be more to come once I think of it, but what do you guys think?