Guild Wars 2

Located between the real world and the mists there is the otherworld, a place where mighty beasts roam.
Each week a different giant appears, in connection with the star signs.
Of course, in the real world star signs do not change every week but in this place time works different.

Upon entering the portal you should join a commander squad, 1 squad can have up to 200 players thus making a raid.
The raid then fights the giant and if successful, grants bones to players in the fractals of the mists (like +5 more agony resistance).
The bonus is active as long as the giant is defended at least once a day.

Fights should be more strategic than fighting simple champions in the open world.
One drawback of GW2 not being a holy trinity game is that players have less dependency on each other.
There is no need for example for guardian and elementalists to work together in a certain way in order to protect the elementalists and making it possible for them to mass-heal.
But perhaps a strategic element could be brought into the fights by some other means.

Players should have a chance the get mists essences for rewards, as well as fractal relics.

200 people in a raid? On the same map? Fighting the same thing? No

You know that a normal map can currently hold 150 players.

200 players doing something together in GW2 is nearly impossible, if not 100% impossible.

Just so you ex-WoW players know, typing in the term RAID (no matter WHAT the content actually is) will not be generally accepted well in a non-WoW forum. Make up a new word if you want existing players to not immediately be turned off by your idea….just saying.