Interesting opinions! Seems like I will have some trouble if I decide to make a campaign that way. What you think of this scenario I was planning:
The main villain will be a angel. He was corrupted by powerful forces, becoming cruel and evil - the opposite of everything he once was. He fell on the abyss, quickly growing stronger as he defeated other demons, as he possessed the power to absorb the strength of those who he defeats.
Eventually, the angel managed to defeat his first demon lord, absorbing its powers. With it, he went to kill other stronger lords, acquiring even more power as each of them fell to him.
At some point he will have defeated the strongest demon lords and
combined the strenght of all of them in his body - a power that could outmatch even a god, becoming the "ultimate evil". With it he united the entire abyss, as no demon could challenge him anymore, and went to conquer the other planes.
The gods started to fear his growing influence, and a few tried to stop his advance. Still, it was too late, and the first god died by his hand, becoming a part of the angel's powers. Those who tried to fight were defeated just like the first, and one by one the planes were being conquered.
Eventually he gazed over the mortal plane, and his endless armies invaded to conquer everything, his lieutenants commanding them as he personally invaded other parts of the multiverse. The PCs will fight against his forces of demons/devils/corrupted beings, and unite the entire world for this war, receiving the help of the remaining gods to resist the invading forces.
In the end they will have defeated the enemy commanders, attracting the angel attention. Then, he will send his most powerful lieutenant to destroy the resistance - Iomedae, the only god who he spared on defeat, corrupting her essence so she would represent the opposite of everything she once was, serving him.
The PCs will, with the help of the remaining gods, have their final battle against her. They wont be able to win this fight, but with their sacrifice will defeat her, preventing one of the continents from being corrupted by the evil forces.
Then, the next campaign will be in that said continent, were the PCs will try to restore the other regions who were corrupted in these events, while finding a way to prevent the angel from causing further damage!
This is mostly some ideas Ive been toying with for the next campaign, a lot of things will happen in between these events too. It got a bad ending, but maybe the pcs can find another way to finish it, who knows.