As far as I can tell, the biggest idea-well here is EVE Online. Have you ever heard of things called Emergent Gameplay or Goonswarm? I think this video can take many of the points I'd like to make right away. View: http://www.youtube.com/watch?v=nvK8fua6O64 Long story short, I want to encourage backstabbing but not on the "lol I trole u" level, I want it strategized and engulfing the whole game world. I want there to be faction divides here, even within the Space Marines or Eldar. Imagine a large part of the Orkz are led by a Blood Axes boss in a fairly negotiable mood and he offers the Space Marines a temporary truce while they concentrate on other problems. The Spae Wolf commander around thinks to himself "sure, we're getting our shit pushed in from all sides here, better take the lesser evil". Except a wannabe Black Templar (perhaps a Dark Angel who had wolf piss in his breakfast cornflakes that day for some unfathomable reason) disagrees and calls the Space Wolf chief a heretic to the cause of the Emperor for cooperating with xenos. Cue infighting. This kind of evolving, smart gameplay would only work out when you get full-on FF activated all the time. Everything else railroads you into two camps, Alliance and Horde and you're stripped of any interesting mechanics inside them. What if I disagree with the boss? Am I just supposed to suck it up then? No, of course not. I'll throw a wrench in his plans and ursurp his ragtag bunch of fighters while I'm at it. For example, you could have a group of Soul Drinkers CSM who want to repent for their deeds or such, they could work together with the Smurfs under the tag of going on a Penitence Crusade, working with the Spacies for the Emperor. I know they aren't listed in the faction lineup but that is what roleplay is for and this setting of all practically begs for roleplay happening in it. The gameworld this would lead to would be one in which you could only trust those fighters you actually know- or at least those fighting under a person you know, actual brothers in arms that work together for this cause because they believe in it and are joined under a feeling of brotherhood. Everyone else is to be regarded with suspicion at best and outright hostility at worst- and isn't that exactly what 40k is all about? Let the games begin and let the players play- the more freedom you give them the more mindblowing the result will be. Railroading is easy, but it doesn't pay off in the long run because that gets boring and requires huge amounts of dev work to keep interesting, see WoW. If you want a game that runs itself, go full sandbox.
Seriously, I understand the appeal of the mass free-for-all sandbox and of emergent gameplay. I was a huge fan of Station 13, and I'm looking forward to its spiritual successor Centration. However, that's not to say that sandbox approaches are automatically better, or that there is no middle ground. Take RPGs. Bethesda put their stake in the ground on the sandbox end - if you want to, you can entirely ignore the main questline and go and do your own thing (particularly in the older Elder Scrolls games). On the other end of the spectrum you have the likes of Bioware, who use railroading and divergent pathing to focus on storytelling. Both capable of producing great games in very different styles, with their own different forms of replay value. Now, if this was a 40k game set in Necromunda, I'd be entirely behind the idea. But I'm not convinced its right for EC, because the game vision isn't throwing players into anarchy, but encouraging them to co-operate in an overarching campaign.
