Hey there, I have a suggestion to a new ultimate raid :-)
I'm sure this would fit in very well, and have a lot of replayability.
This ultimate raid would only be available after completing the Hildebrand questline.
You start the raid by asking Gilgamesh about why he hunts for the weapons.
This raid begins by Gilgamesh telling the story of his hunt for the legendary weapons.
You will fight against all the bosses that possess the legendary weapons and Gilgamesh stealing them at the end.
At the end, Gilgamesh would forget to close his space rift, following him you reveal that his base of operations is actually a highly technological spaceship, and that he knows a lot about the worlds, reflections etc.
But going through too many dimensional time rifts has fragmented his mind and made him crazy and given him a very bad memory.
Also, perhaps the ancients created Gilgamesh with the purpose to hoard the greatest weapons in the world, to prevent them from being used for evil, but actually ended up dooming the world (again).
So, the final boss (Gilgamesh) would be him wielding a combination of 8 weapons that may or may not be legendary, depending on the outcome from your previous fights.
All the 8 first fights would have the possibility to give Gilgamesh different weapons, and here I would actually like to introduce a randomness to it, based on his poor memory.
So, the 8 first boss fights that will determine the final weapons of Gilgamesh, will have a chance to give Gilgamesh a RANDOM legendary weapon.
Narritively, for comical sake, it could be presented that Gilgamesh doesn't remember which weapon he got from each boss ;-D
And the fight mechanics would also include tackling environmental/perception issues, like for example, "Hmm, I don't remember those flowers were blue...". And then you would for example have to find red or yellow paint to paint the flowers in the correct color... ;-D
So you would also have to assign someone in your group to take care of scenery.
Or maby he didn't remember there being so many people there, so then maby 4 people needs to hide from plain sight...
Or maby he remembers one dancing on top of that pillar ;-D
All while fighting a boss, like Odin :-D
Each legendary weapon he doesn't get gives him an enrage stack.
If he doesn't get any legendary weapons, he summons Enkidu that wipes the group.
Each legendary weapon gained by Gilgamesh will give him the possibility to cast the weapons ultimate ability.
If given a fake weapon instead, it will do less damage, will be less complicated and easier to dodge.
Ultimate combo attack: perform all 8 legendary weapons' ultimate attacks in a RANDOM order maby symbolized by the weapons start glowing in different order.
At 50% Enkidu joins the mix. Gilgamesh also employs cheap tricks to catch you offguard.
Backdoor skill:
Will travel forward in time to perform random ultimate ability.