Yvraine gets The Lion off his ass and a Dark Angels vs Ultramarines cold breaks out war over which Primarch gets to claim that Eldar booty. Loser gets relegated to the newest toy in Lelith Hesperax's dungeon.

On a more serious note I'm hoping for one of Perturabo or Lorgar returning while Mortarion starts doing stuff outside of trying to troll Ultramar. At the same time The Lion wakes up and plot armors his way into the Dark Imperium and takes some heat off the Blood Angels.

In public they would have a unified front about who goes but in private I could see some rather heated arguments with the Lion breaking out the Imperium Secundus argument about what happened the last time Guilliman thought the Imperium went dark, and how things could very easily develop into a massive civil war if the rift is overcome and the two halves of the Imperium meet up again.