End game content

Well, this is a terribly difficult idea to implement

One of the problems why players "have nothing to do" is that hand-in quests are completed too quickly. And this is already being considered.
Generally speaking, end-game missions are possible to create, but it's definitely not easy.
Now about cosmetic prizes. From a P2W perspective, it would be a good solution. But do players really want to take on extra load just to get, say, one more star added to their existing 8?
Regarding quests that change every season. It's also possible, but very complicated. Believe me, I know. I tried creating hand-in quest generation based on this principle.
As for another game example, better via ticket (dc pm disabled).

One of the problems why players "have nothing to do" is that hand-in quests are completed too quickly. And this is already being considered.
Generally speaking, it is possible to create the mentioned end game missions, but it's definitely not easy.
Now about cosmetic prizes. From a P2W perspective, this would be a good solution. But do players really want to burden themselves further just to get, say, one more star next to their existing 8?
Regarding quests that change every season. Also possible, but very complicated. Believe me, I know. I tried creating hand-in quest generation based on this principle.
As for another game example, better via ticket (dc pm disabled).

Regarding the difficulty in creation, I fully understand, so perhaps with a bit more time something could be achieved. As for player engagement in this, of course it's hard to say exactly what the demand would be, so it could be possible to add some kind of item reward for completing everything or some benefits. But essentially my idea would be just some kind of title, which definitely wouldn't be easy to obtain, and which would show that in this segment, the player truly knows what they're doing.

Well, this is a terribly difficult idea to implement

I don't know if the solution is any easier to implement, but I've seen this in a lot of games:
Prestige (ascention, rebirth, loop - call it what you want, it won't spoil it)
The idea is that when you reach the last rank, you can be reborn and start again from the first rank. For rebirth, you get some small boost, accessory, or icon. That's up to you to figure out.
The point is that you can raise the rank endlessly.

Well, this is a terribly difficult idea to implement
I don't know if there's any slightly easier solution to implement, but I've seen this in many games:
Prestige (ascention, rebirth, call it whatever you want, it won't spoil it)
The idea is that when you reach the last rank, you can be reborn and start again from the first rank. For rebirth you get some small boost, accessory or icon. That's up to you to figure out.
The point is that you can raise the rank infinitely.

But apart from constant grinding, how else would this be useful to the player themselves?