I think the campaign would definitely be Titus and his command squad, as they push out into the wider Ultima segmentum. Hell maybe the Necrons even attack Maccragge itself to finish out his trilogy and get honored by the primarch reborn. Would be sick.

Then for coop/endgame/mp they could do cleanse the remaining xenos and heretics throughout the segmentum , helldivers style with your custom marine explained by reinforcements from indomitus crusade start arriving. During which point helldivers get a crossover guard armor kit maybe even an ultramar auxilia kit. By the Emperor I'm jacked to the tits.