Space Marines would best be represented in a Doom-like one versus a bazillion style game. Really Darktide being Guard is a bit of a loss. 4vHorde coop is perfect for Astartes. 4 flashlight boys taking down hundreds of grots is a bit of a push and I'm sure they'll be gunning down boyz and nids at that rate.
I think a Battlefront/field style game would be great for IG/Orks/Mechanicus/some variations of Chaos/Tau. Astartes would be hard to balance, other than as a general Imperium "Heroic" call in like in Battlefront. I think nids would be harder to balance unless it was a lot like Battlefront with large armies of trash NPCs and you're starting above -guants. Basically 40k Tarpit battlefield with the bigger boys being call ins.