It would be great to have world bosses spawning around the game world. A few in every zone. These would require a few players to do. Each player could kill the boss for loot and experience points once per 4 hours, but no more than twice a day. Every boss would respawn every 5 minutes. It would also be marked on the world map with a special boss icons so players know where to go to kill it. It would be a kind of an optional daily task that players can do if they want. Each boss would give a good amount of xp and a chance at obtaining some cool items (like 3% drop chance). Not necessarily the best items in the game for end-game content, but more of an entry level items for starting doing the end game content. + maybe each world boss could drop one of the best end-game item but with a special prefix called "World Boss Trophy" and you could only have 1 or 2 of these items equipped at the same time so that it would not be possible to gear your character just by doing this content.
I would definitely enjoy logging into the game everyday and hopping straight into action by forming a party and going to every zone to kill some boss monsters.