There's these two ideas I've had rattling around in my head for a while, that I think could be interesting alternatives for worldbuilding or stories.
The first is the one I've been thinking about for longer. This is related to the existence of violent former demon lord era monsters still living in small numbers even after the rise of the current demon lord. This might not be very acceptable into the canon of the world, but I want to offer it as a possibility in the case that others like it.
KC doesn't tell us how mamono changed from their violent to their loving selves. Ore-Samma posits in his write-up that monsters changed immediately, without any memory of their former lives. I think this is fine, and works within the logic of the mge. Demonic Energy is powerful enough to affect such drastic changes to people and monsters.
However, I have a different idea. I think the transition could have been a bit more gradual. I imagine a case where monsters recognized the change of rule, but not all of them were willing to immediately adapt. As mamono began to integrate into human societies, there is one final generation of monsters who, despite their body changes, still remember their lives as violent killers. Some might be willing to take a husband, but some might prefer to pass on, preserving their own memories of their past lives. This allows future generations to make proper alliances with humans.
I developed this idea with a few story ideas in mind. There may be monsters who are continuing to kill, refusing to submit to the demon lord. So she puts together teams of mamono who are meant to find these more dangerous monsters and convert them into proper mamono.
The witch of Hansel and Gretel fame lives in a candy covered house still trying to lure in children from the nearby village. The team needs some bait. Can a sabbath witch fool her?
A monster makes travel nearly impossible as any ship traveling through a stretch of the sea is attacked and dragged beneath the surface.
A lich has a hold over the entire region, terrorizing the people with waves of invading skeleton forces. His armies grow smaller, and now he has hid himself in his castle, putting an end to the attacks but making it difficult for anyone to enter and break his control over the land.
The other idea I had was of a girl and her partner getting together, and her wanting to introduce him to her grandmother. Grandma is one of those man eating monsters who's resigned to passing away. I imagined in this case it could be a siren. It's like, "let's go meet my older relative and talk about different our lives are between then and now, except in her case it's about how she used to eat people. and she's happy for her granddaughter finding a better way of life". I liked the idea of the couple and the monster (it could work for many different species) having a talk about how their lives have changed since he change of demon lord.