Okay, it's decently known that im not the biggest Aeldari fan, but one thing I've been thinking about a lot recently is an older post @moociao-after-dark made regarding Isha (Eldar goddess of healing)
She said that the Eldar should be the ones to save her. I completely agree. Because I've been thinking. So after the plague war, Mortarion gets imprisoned in Nurgle's garden, getting punished for bringing Guilliman/Emps into the garden and getting it burned.
But what if Isha witnesses all of this, and now she can see the hurt and anguish plauging Mortarion's every waking moment. She may have already known or just realised that Mortarion is one of the only truly unhappy,spiteful, reluctant followers of Nurgle.
So when the Yanari or whoever come to aid their goddess in escape, Isha takes Mortarion with her. Adopting him, showing that despite not being of her people and after all the pain and suffering he's caused, he's just a traumatised little boy who's never known a gentle loving touch.
Morty is still morty, though, so he's petulant and sullen, but he feels himself healing. He feels the state of decay and disease being purged from his system. He feels the filth and grime get washed away. He blooms into something altogether new.
He still despises the eldar because of his deeply ingrained xenophobia and aversion to psykers, but he doesn't just slaughter them. The Eldar question every moment why their goddess saved this mutated monstrosity, but he is under the devine mother's protection.
Those of his sons who can and want to follow their father also receive Isha's blessings. Turning from the Death Gaurd into the Bloom Guard ( This last bit is mostly inspired by the Femmarch AU from the official PrimarchGF Discord. We take absolutely no credit for the bloom guard name/premise on the above story idea)