Maybe a dlc idea but how about 1ce a week a random zone in tamriel is flooded with powerful daedra with a big shiny portal at the end of it.
Only 1 wayshrine is active in the zone, and the aim is for guilds to battle through the zone to reach the portal at the end. A nice large no combat restriction around the wayshrine to stop wayshrine camping.
Pvp toons have to defend the pve groups from opposing guilds as all the mobs throughout the zone will need tanking (not your average mobs) then once met at the portal the guilds strongest pve 12 man group is then instanced into the heart of hell to face 1 of molag bals generals, once victorious the zone is claimed in the name of the guild, and each day the portal is under control of the guild a chest inside gives out a gift from sheogorath,
Its up to the pvp community in the guild to defend the portal entrance from opposing guilds, while the pve groups can go to the already existing dolmens(with greatly strengthened mob spawns) to kill other generals to increase the difficulty of the boss in the portal, kinda like a cloudrest plus 3,
Obviously this wouldn't be alliance restricted, each week you have to decide as to which guilds flag you want to fly in order to enter the zone,
Rewards can be 1 piece of the sets from that zones dungeon/overland but maybe known as daedric infused, giving an extra 6 piece bonus, for instance if the portal spawned in rivenspire the ebon armour could have an additional 6th piece which gives minor protection to everyone in the group at all times, the lich set could have it's 6th piece as automatically applies magicka steal to all enemies within a certain radious,
These sets would be bind on pickup as to make sure they took time to get together,and obviously only lootable 1ce per day per account. Obviously being rather potent but tradeable with other party members for 2 hours from pickup. So guilds can decide for there trial team all of 1 particular item set can go to 1 tank or to 1 healer dps may fight over the scraps. Pvp have to look after the pve folk to get there sets and the pve folk have to do their job so the pvp efforts outside aren't wasted.
Achievements and titles are endless, everything from killing 100 people in a daedric infested zone to everything pve related, crown store can benefit from selling dyes, mounts, outfits for all of your guild to look all pretty and unified hell you could even sell full group ressurection stones for 100 crowns a piece(only usable in the selected zone for the week)
I think this would be a nice idea