I thought of an interesting idea for a new4-person arena which I think would be an incredibly interesting experience and there currently isn't anything like it in ESO.
Premise: A daedric ritual is commencing to summon a world-ending beast (ok this premise isn't so original) and only you and your group can stop the hordes of daedra from sacrificing themselves to summon their master.
Set-up: This is a timed arena where the goal is to survive for 30 minutes from an increasingly aggressive and powerful horde of daedra. The catch? The daedra won't attack you, if you don't attack them. The daedra instead are rushing to sacrificial blood alters scattered throughout the arena. As the dadrea spend more time at the alters, a circle of poisonous blood (touching the blood will kill you) encloses the arena. The blood starts as a ring on the outskirts of the arena and moves in toward the center. The daedra's power increases as the timer counts down, as does their frequency. They spawn from the edge of the pool of blood, which means the more arena floor that's open, the more daedra that will spawn. Once the alter they are at is covered in blood, the daedra will die, which will prevent groups that are doing too well from suddenly being overwhelmed if they begin to falter. On NM, the arena is done if the floor is covered in blood or if you survive for 30 minutes. The award you receive will scale based on how long you survived. On Vet mode, instead of ending after 30 minutes, a final boss is summoned to fight. At this point you have to switch from killing things near the alters to activating an altar to be able to damage the boss. While the alters are activated, the floor will continue to fill with blood. This means that before the boss fight, you will have to decide how much floor you want remaining. It will be easier to work together if the floor is small, but you will have less time to kill the boss. Unlike other arenas, when you die you can self-revive with a soul gem, instead of needing to be revived, but the extra time it takes to be able to fight again will obviously hurt the group.
I'm not an artist, so you will have to use your imagination for the final version. The black space along the edges could be the set dressing, such as alters, artifacts, statues etc. etc. The stars are different alters. They are grouped by color to show which alters will be targeted together.
Strategies: As you can see in the picture, at first everyone must work separately to defend and alter from the encroaching enemies. This means that a group's healer and tank must be able to do enough damage that they can sustain holding off the hordes long enough so the team can converge to the next alters once they are overrun. The second set of altars are in pairs so that people can work together to defend. At this point, there should be fewer monsters that are stronger. Likely each altar would need a dps and a support. After the enemy hordes become overwhelming, the group would fall back to the last set of altars, which are next to each other in the center. At this point, the tank will need to act like a tank and pull larger monsters off the altars while the dps burn them and the healer keeps the group alive. Hopefully, by minute 25, everything is just chaos.
Rewards: Standard rewards, such as gear set pieces with a chance of a motif chapter. The chance of a chapter/quality of the gear has a chance of increasing with each 5-minute segment. If you can beat the final boss you get a random weapon that is similar in quality to master/maelstrom/perfected.
Advantages: Not only do I think this would be really fun to play, it would probably be fun to watch someone stream. This has the possibility to have different modifications added, like a mode that uses random altars instead of the set ones or a mode where the circle always moves in and the monsters target players and they just have to survive.