I am sure it is too late for this sort of thought in the development of the game but I plan to get really into this one long term so I want it to be really cool.

Looking back on all of the MMO's Ive played I realized that there is something that hasn't been done in a long time but really made games fun so there was always something to do. I am going back to Ultima Online and specifically the ability to declare war on other guilds and fight them in open world combat. The rivalry was really fun, and usually moved over to the message boards to talk smack. If people had a hard time playing the rest of the game without enemy guilds attacking them they were forced to switch guilds or get theirs to declare peace. If you were hardcoe you had to find better and better guilds to suit your play style. Like you had to get into a guild who could watch your back, and if someone killed you and camped your body you just had to call in your friends. We never got much for the kills but it led to a community feel where you knew just about everyone on your server. Bragging rights were all we needed. The guilds had colors you always had to wear and you could tell which guild people were in by their appearance. As far as I know no game has done anything like these points since UO. All of the other MMO's out are too cookie cutter and easy to play because everything is set up for you.

The thing is the devs have said there is no PvP with your own faction and you are already at war with the other two factions....I like the idea too because I remember that from UO as well but not sure if they will consider it for ESO.

The idea isn't that bad, but we have another PvP model here. Fighting your own faction isn't this wise so you can declare war against a guild of the other faction, but didn't you fight them even if you ddin't declare war?
Like i said nice idea, but not in this game.

Dark Age of Camelot had something like that. It was FFA server "Mordred". You could atack anyone exept your guildmates and alliance members. You could move trough all 3 faction zones enter all dungeons and guilds could have any matchup of all 3 factions players inside.