You can have cross-faction guilds, but only in bubble worlds like instances, as we’re seeing. (Well, you could have a “cross-faction guild” where people can’t trade or group or quest together, but… doesn’t seem like much of a guild, to me.)
I’ve been explaining this to people for a long time, about how Blizzard can do whatever in instances because they’re split off from the rest of the game. But factions are so hard-designed into the open world that you cannot have cross-faction without removing the factions entirely.
And people kept telling me I was wrong, and then lo-and-behold, cross-faction comes out with exactly the sorts of limitations that I was telling everyone about.
And people STILL tell me I’m wrong.
I just wish people other than me would actually think this through for once.