About Primarchs, I don't know if it's cut and dry that they're going to alternate loyalists and traitors. We only know for sure where 2 loyal Primarchs are, Guilliman and The Lion. There are a lot of others that are /probably/ still alive, and could return in theory, but they're all basically Schrodinger's Primarchs until GW
lets us know that they're alive or dead. Whereas with the traitors, we have GW
confirming 6 are alive, one is unquestionably dead, one is dead with uncertainty, and Alphariusomegon is half dead and half unconfirmed. So if they're going to release 18 models, alternating makes sense. If they're going to release 8, then it may be very traitor oriented more often than not. Now I fully believe that we'll see more than just Guilliman and The Lion, but who knows what their ultimate goal is.
And about The Lion, a lot of people seem to be sure that he'll oppose Guilliman. They were both part of the triumvirate in Imperium Secundus, and the only conflict I could see if it they both demand to be de facto Emperor. I can't see The Lion being upset over the Legions being broken up because, well, the Dark Angels never truly broke up. If anything, I feel like we may get Imperium Secundus Secundus, and the real friction wouldn't come until they brought back a third Primarch to reallllly oppose them. Of course, I'm as likely to be wrong as right until GW
makes a move.