Warning I own nothing and I am still learning about the era of the great Crusade so for now this is going to be a small concept if you have any notes please tell me leave it in the comments or something.
The basic premise of the idea one of two options either someone dies and becomes Primus or Primus dies and gets brought to the 40K world before Horus is found in the great Crusade and there may be Optimus Prime and or Megatron that's questionable but the 13 primes will be there and since we don't know exactly the 13th prime is besides what from IDW comics incident in their story where Optimus is the reincarnation of the 13th so I'm going to use Transformers 1 and say Zeta Prime and Unicron will be in this as the as the chaos God of chaos gods with his emergence there will be a new Chaos Faction will they be demons will they be cemeternians will they be more Space Marines I do not know I am still figuring that out and I plan on releasing the first chapter in like a week or two.
On an unrelated note to this story I do plan on continuing my Transformers Prime story but I haven't technically started I've had to rewatch the entire show and I had to revise it a few times.