Unlike the other two Gathering Storm books which were quite apparent in how the fluff was going to change, Rise of the Primarch is the most up in the air and will have the largest impact, in my opinion.
I want to know what people are speculating will happen in the next rendition of the Gathering Storm. And I am not talking about the obvious stuff like Guilliman being revived and getting his hands on Cypher. Let's look at the possible ramifications and see how close we can get to the truth when the book is released.
For myself, the biggest thing I can see happening is the Unforgiven going to war with the Sons of Ultramar. Why, I hear you asking? Well, maybe not asking because you can probably already figure it out. Anyways, the information we have been given is that after Guilliman wakes up, Cypher asks for a ride to Terra, and the images they released show Cypher joined by several other Fallen. So, right there you have another Chapter that has Cypher and friends willingly in their company. The Dark Angels have made Templar Crusade fleets disappear for having one of the Fallen. Do you really think that Dark Angels will allow the Ultramarines to have access to the physical representation of their shame? Not freaking likely.
Because of the unusually close ties the Successor chapters have with their Founding chapter, the Unforgiven have often been accused of Legion building. The Chapter Masters of the Successors, if fighting in the same campaign as the Dark Angels, will often defer to the Supreme Grand Master; and there is only one Supreme Grand Master in all of the Unforgiven chapters.
So, when I say that I have a feeling the Unforgiven will be going to war, I mean that Azrael will call all of the Sons of Caliban to his banner and he is going to lay the boots to the Segmentum Ultra - Primarch or not.
That's my biggest speculation about what is going to happen. What does everyone else think will be going down? Why does Cypher want to get to Terra so badly? Why was Guilliman revived now? So many questions... so few answers!