Do not go gentle into that good night,
Old age should burn and rave at close of day;
Rage, rage against the dying of the light.
Though wise men at their end know dark is right
One of the things The Book of Cain mentions about using the World Stone is the fact while it has been used many times, the worlds created ALWAYS die. Whether created by Angels or Demons.
Now while Inarius was able to do some things no one had ever been able to do before, such as hiding it from the view of both Heaven and Hell -WHO is to say that Inarius "Sanctuary" is any more stable a world than any of the others created?
We dont know how long these other realms have lasted or if any are still around to guess what the lifespan of sanctuary might be.
WHAT IF............The End of Days is JUST beginning?
If Sanctuary is nearing the end of it's lifespan and REALITY begins to break down WHO could repair it?
The worldstone is gone.
And for it's destruction, many of humanity might be angry at Tyrael for taking away any hope of repairing the world. Perhaps, indeed, they might have prefered the moral corruption of Baal to TOTAL dissolution.
Tyrael might advocate in such a situation that Heaven allow humanity to be rescued, but while there may be no lack of space humanity is NOT harmonious and Imperious wouldnt trust the "Nephalem" not only for their chaotic and immoral behavior, but also because he would FEAR their ever increasing power with EACH generation.
The doors of Heaven barred to them.........what if..........................................
What if someone like Adria reminded them that "the gates of Hell are always open."?
The Demons may want to simply END their war with The High Heavens, even under the pretense of allying with humanity and simply "giving it away."
But with the lords of Hell ALL vanquished who remains?
A hanful of Azmodan's lieutenants and maybe......Lilith. She'd escaped from the black abyss of the void before, perhaps she might return proclaiming herself "true mother of all mankind", both a demon and a goddess.
Where the player characters come in, is in trying to put down a violent uprising against the heavens fighting both empowered Nephalem and demon alike, while also trying to fight their way into the deepest reaches of Hell....to seek answers. If their is ONE person who would know how what made Sanctuary flawed -it would be the man or rather Angel that created and to this day is still tortured by demons -Innarius.