Its been speculated that guilds in ESO will be multi-faction. Meaning you could have DC, AD, and EP all in the same guild if you want. There might even be some Dev quotes stating such but have also heard the opposite.
Anyways purpose of the thread isn't really to discuss if its true or not. Reason is, based on what we know of the game, I'm not at all sure what the purpose would be of a multi-faction guild. We know we can't PvE group and quest with an enemy faction in our own faction lands. We know that the only time we will even see an enemy faction players is in Cyrodiil and there they will be hostile/enemy targeted.
So question is, beyond sharing a guild chat channel what is the gameplay purpose of having a multi-faction guild. Because to me the way the game is set up there is no purpose or mechanical gameplay reason for it.