Too many good idea, we need to take them all :

1. First the Players will have to assemble some Huge Cannon and prepare their ambush on the Whale, when the Whale is finally in reach. There will be a fight, where the Players attack the Whale from the ground, while the Whale send minion, that are attached to her body, down to destroy the Cannon.

2. Once the Whale get hit enough, she get tired and need to land.

3. There it'll be a Huge World Boss with terrible AoE, the fight of the year.

4. Finally the Whale is knocked unconscious, Players will be able to run inside her mouth to enter an Open World Instance.

5. It'll be a race against time, you need to clear the instance while other Players are in your way and before the Whale gain consciousness again.

6. Once the Boss is reached, one Guild will have to "Tame" it, the Guild which will be able to do so will be seen as stronger than the Boss and be allowed to inhabit inside the Whale, while everyone else get kicked out.

7. The winning Guild will now be flying around with their Whale House, which they can now upgrade like a normal Guild House, but worst ! They can even add Turret and Cannon to the side of the Whale, to rain fire on the land below.

8. Now it's back to square 1, more people need to build Cannon and prepare an Ambush for the Whale, so they can kick the first Guild out of their house and claim it for themselves.

9. ????

10. This can't go wrong !

Giant Whale Guild Hall vs Giant Whale Guild Hall Seiges... O.O

+1

please make this a world raid boss.