Whilst the White scars are in desperate need of some background, I would love to see some alliances between Legions during the GC, but not just a out and out war story, I'd like to see the 1k sons fighting along side the Alpha Legion, during the midst of a campaign trying to discern the Alpha Legions motives through warp practices, and the Alpha's being more secretive than ever as a result. Think on at how awesome that could be, the level of distrust would be immense, you could have Alpha Legionnaires assassinating 1k sons and the like for unknown reason, Magnus not taking action, even though he knew it has happened because something has been foreseen..ect ect ect..
I know technically this is nothing new, so in that regard I'd love to see some more on thunder warriors, the primarch project, some more primarch history,
"The difference between gods and daemons largely depends upon where one is standing at the time." - Aurelian Lorgar