First of all, I think that the main problem of Diablo 4 is the lack of an endgame. Everything comes easily and reaching the maximum level is not a problem.
I won’t write about the disadvantages here, because everyone knows them, so let’s get to the point.
At the maximum difficulty level, we got access to a tower with infinite floors, with different monsters on each floor and their number depending on progress, and a boss fight every 10th level.
I think that the rewards should also be some weapon or armor improvements, e.g. the ability to make weapons or armor at +5,10, etc. However, with a small chance, upgrading all armor parts to +5 would give other defense bonuses, e.g., there is room for improvement here.
In tower, we can test ourselves and our character and we always have this carrot in the form of challenges and improving our character.
I would also add a clan hall system since clans are dead now. In such a system, everyone could meet in one place, and in this place you could receive some reinforcement for half an hour for experience or defense, but you would lose it in the event of death.
Additionally, there could be some clan missions that would give bonuses or decorations, etc.
It’s all so simple that I’m surprised no one has thought of it yet.