I would like to see world boss events, such as major boss fight, whom are requiring a lot of time, coordination and stages to bring those bosses down and defeat them. What I'm thinking about? Lets introduce one ultimate world boss. Lets say we have one Dragon ultimate boss. This Dragon, unlike the others, should only appear once per day / week / month / quarter / year and defeating him should grand the lucky players with a very imposing dragon treasure consisting of several random blueprints, recipes, motives, large amount of treasure (items), large quantity of gold and experience, large amount of crafting materials, artifacts, set items coffers (full sets), unique monster sets, unique furniture, or even deeds for a new homestead. Rewards should be tiered and based on how the boss was dealt with, the more damage it cause the less treasure we get. The faster we bring him down the more treasure we get. Achievements could be granted to every player who participate depending on the outcome of the fight.
There should be two types of events, once per day ones and once per week / month / quarter / year ones.

But lets talk about the fight itself.
Spawning... I will let you figure out.
Fighting however, should be taken down into several stages that are spawned during several days. Lets say that boss will spawn in Stonefalls and then slowly moving towards Deshaan, Shadowfen, Eastmarch and the Rift, over 5 different days... In each zone the boss should move from the spawning point towards the exit point and kill and destroy everything that stand in its way. Your mission should be to prevent this boss to reach and destroy the main cities from each zone, Ebonheart, Deshaan, Stormhaven, Windhelm, Riften, for this particular example. Once you deal enough damage to the boss, he will no longer head towards that city but instead just run towards the next zone by vanishing into the mountains, sea, air, undergrounds, space, time, etc.
During the fight itself, the aggro area should be increased by lets say 10 times, and hordes of creatures or just mechanics should affect all players in his path.
The main boss could be a dragon, surrounded by 3-5 lesser dragons, all flying at low altitude, allowing players to range attack the boss, but never go into melee.
Or boss could be a huge Atronach, immune to any sort of ranged attacks, all by himself or with a horde of enemies accompanying him.
We could also have daily world bosses, bosses that spawns once per day per zone, such as a Dragon in Northern Elsweyr, with a 3 stage fight. First stage in the north, moving from one battleground to another. Second stage should be in the east, again moving randomly between battlegrounds. Last stage should be in the south.

Since we were talking about world bosses it would be nice to have some daily world boss / delve quests into all major zones from Tamriel, and not only chapters / dlcs ones. On top of that there should be a quest that spawns once per week and ask players to lets say kill all world bosses from that zone or clear all delves from the zone in a timely manner. Daily / Weekly public dungeon quests should also be introduced for players willing to spend some time in some more challenging ways.