Based on what the warp seems to have done to him, Corax. He’d be a great foil to the more strategic approach of Guilliman (e.g. fine, you fight your wars, but I’m taking my chapters to lop off the enemy’s head), and I can envisage a cool model where Corax is shape shifting into a flock of ravens.
People here keep repeating 'foil' and I just wonder if anyone bothered to read Dark Imperium books. Rob mentions like a hundred times he would love if any of his brothers took some of command roles off him so he can focus on shoring up rotten foundations instead of wasting his time on petty campaigns. If Lion came back with his dumb fixation on getting the warmaster title G wound literally throw the award diploma in his face and told him to start yesterday. Foil? What foil? Galaxy is large, even if returned primarch wanted to change the warfare style he could just pick another segmentum to never even see others, never mind actually arguing over anything with anyone.
I would really like the loyal Fulgrim to return from the Fabius books. It would create a nice triangle between Gman and daemon Fulgrim. At the same time warp corax returns. The contrast between a loyal primarch who's form has changed potentially and a fallen primarch who has (through cloning) returned pure.
Honestly, I'd love that to be the case. Loyalist Fulgrim would be unencumbered by stupid baggage most of the snowflake legions picked up over last 10000 years, while needing to constantly prove himself and living with doubts far greater than any other primarch and trying to make up for the actions of his possessed body. It would be one of the most potentially interesting returns which is sadly why it's one of the least likely to happen...