The Horus Heresy: Inferno

[Dînadan]

As a Thousand Sons fan part of me would like to see Russ return as a slavering wolf creature, but realistically I think that'd be a misstep.

Realistically I think Lion will be next considering Cypher has now shown up working with Guilliman and the likely advancement of his arc. Although no idea if he'll stand by Guilliman's side or if the Angels and their successor chapters will cede from the Imperium as a result.

Beyond that, I think the best one for the Imperium would be Dorn (assuming he doesn't butt heads with Guilliman) as he's the Builder of the loyalist Primarchs and is the one most likely to try sorting things out and rebuild rather than tearing off into battle.

[Fixer]

The Lion would certainly be more interesting, raising questions of his true allegiance during the Heresy.

The more unconventional primarchs would also make a nice contrast against the extremely straight-laced Gulliman.