Since GW is bringing back Guilleman you now have someone who was very likely to know what happened to the missing two Primarchs. Since GW seems intent on running the end times, it implies wrapping up story ends by marching to an end. What happened to those two is kind of a big lingering plot thread. Do you think GW will change decades of keeping them as a throwback to the lost Roman legions and so players could claim their unique chapter as one of the original 20? If they were going to, now would seem like the time and I for one think they should, even if it is a throw away explanation that Guilleman says they were sent to slow the Nids so the Emperor could unite humanity and prepare for their eventual arrival. Now that the Nids are here Guilleman can speak of them without causing panic, que info dump.

My own speculation on if they were actually return to the universe as a playable faction would be that they wouldn't necessarily be imperial due to Guilleman's new marines. On the other hand, making them Chaos would really upset things on that side as they likely were Chaos before it was cool. No one wants hipster SM. IF that were to happen I would hope that they would be on a third side different from both, even though I'm generally against more marines.

On a similar note, the Cursed founding eludes that an attempt was made to make new Primarchs and that one might have actually survived and was whisked away by someone. Do you think this possible Primarchs might show up? I think it would be a better way to shake up Chaos with something new than one or both lost legions. What are your thoughts?