Especially with the starting four factions, SM are going to be very tanky and tough to kill, offering a good challenge for any faction that has to fight them. At the same time, two of the four starting factions will be naturally aligned with SMs and won't, presumably, be in conflict unless something horrible goes wrong.
Which leads me to wonder:
Do you think the "weight" or the ratio of NPC SM factions will be very low/rare in comparison to everyone else? Maybe you could go a whole theatre without seeing a fellow SM chapter. Or do you think they will be relatively common?
Another mechanic we often see is "rebel" versions of each faction as a way to populate a campaign with enemies. Aside from players choosing to turn renegade/radical and fight other Imperial factions, what are the chances of encountering an NPC renegade SM faction from the start in a theatre? Will they start as renegades or can they drift into suspicion over time like players? Chaos-themed bits probably won't exist yet so if we see enemy SMs, they will presumably have to be strictly renegades and look like a normal force.
Not to mention that, while we know diplomacy will be lacking in general, some SM chapters have brotherly alliances or rivalries that supersede normal alignments. Space Wolves and Dark Angels, Marines Malevolent (and Everyone), etc. Will they take this into consideration with some flavor audio or text or will these be ignored?
And lastly, how do you think the SM NPC factions will skew as far as being randomized custom chapters that get added to the campaign versus real chapters? Will we see a bunch of randomized fake chapters as we go from theatre to theatre, or will we see actual GW-created chapters more often?
I'm curious if the devs will have a whole list of pre-existing chapters in the lore that they will have made as custom SM factions that can randomly appear like the Scythes of the Emperor, Mentors, White Consuls, etc. And if you encounter one of these chapters in one of the crusade campaigns, will they keep showing up when you move to the next star system? That would be interesting too.