I was more getting at the point that a Loyalist likely wouldn't kill himself, unless he was taking the enemy with them. Killing one's self is admitting defeat, and admitting defeat *is* heresy.

True, but keep in mind the devs are letting you shoot in the bleed out state soon, etc. They are trying to be fluff friendly.

Maybe make a compromise where Eldar can be in with Space Marines and Orks with CSM? they wouldn't be friends by any stretch of the imagination, but there have been battlefields where they worked extensively together.

Sub-factions sure but Factions being in a guild with other Factions is a no. They're enemies with their own objectives and desires to destroy everything about each other with the ultimate goal of ending the others existence.

If you want to do that then run with third party systems ie guild websites. Just please don't do anything to break things like telling your ork buddies what's going on while you're on your LSM for obvious reasons. Guild assignment will likely be character bound since that's the simplest & best way to do it and how other games do it. Factions also have their own campaign objectives. So why have factions share a guild when that's the case? To have spies and saboteurs is the only answer.

There will be an alliance system though via War Council player politics just not "Hey greenskin let's join a guild together. C'mon and hang out in our social area. We're pals!" but more like:
Faction A: "Hey c'mon and help us beat back these roughians. We'll help ya later."
Faction B: *hate filled glare* "Fine. But only if we get _ now AND you help with something else later."
or even
Faction B: "F*** you! You lot did terrible terrible damage to us before. By the way, how's the green tide? I hear it's great this time of year."
Faction A: "Oh you son of a-" *communication terminated* "SO MUCH RAGE!"

Exact details remain to be seen much further down the road. I figure it will be temporary and only active per battle/territory. I also wouldn't expect players squadding up cross faction then going to war. Or entering each others vehicles. Would an Eldar even be able to take Apothecary heals? ... Alliances are going to be complicated damnit.

Could a multi guild system work? Final Fantasy 11 had it where it was basically used just for chat channels attached to items called Link Pearls and 1 player could have multiple link pearls but only 1 active at a time, for EC it wouldn't be bound to an item. Could at least act as a comm bridge between guilds who frequently run joint ops together or like have an option to make a shared chat channel that's password locked to do that.

----Off Topic but along the same lines somewhat---
This made me wonder if there could be a universal friends list. It wouldn't allow contact between opposing factions nor tell any details other than "<Profile name> is playing their <Faction> <insert character name>. It would help keep in touch but 3rd party systems can do that to some extent, such as Steam. There could also be unintended tellery of happenings. "Oh, my friend _ logged into their Eldar. That character is in _ guild. Something must be going down. Hey my friendly neighborhood slaaneshi crazies! Time to party!"