The archangels can be really cool characters in fanfiction. They have their own hierarchies and powers. A fanfiction could explore their politics, their relationships with each other, and how they view the world of humans, angels, and demons.
There are fanfictions that rewrite certain plot points in the Angel Sanctuary universe. Maybe a story where the different factions of angels and demons interact in a more peaceful way than in the original. This could lead to new alliances and romances between characters that were previously enemies.
To write a good Uriel character, first understand her basic traits from the Darksiders source material. She is a strong and fearless warrior. So, in fanfiction, keep her combat skills and determination at the forefront. For example, when facing an enemy, she should be decisive and act quickly.
One common theme is probably heroism. Given the 'War' aspect, the characters, including Uriel, might be depicted as heroic figures facing great challenges. For example, they could be saving a world or a group of people from destruction.
Sanctuary can also be shown as a community. In some novels, a group of people come together to form a sanctuary. They support each other, share resources, and create their own rules to keep the peace. This shows the social aspect of a sanctuary.
One possible scenario in such a fanfiction could be that War and Uriel start off as reluctant allies. War, being the Horseman, is initially focused on his own mission. But as they face various challenges together, they begin to notice the strength and determination in each other. Uriel is impressed by War's unwavering resolve, and War starts to see Uriel's more compassionate side beneath her tough exterior. Their relationship could develop gradually, with small moments of shared understanding and mutual respect, eventually leading to a deep romantic connection.
You can try some popular fanfiction websites like Archive of Our Own (AO3). It has a large collection of various fanfictions, and you might be able to find some great 'War and Uriel' ones there. Just use the search function on the site.
I don't know who the author of 'the sanctuary novel' is. There are so many novels out there and without more information, it's difficult to determine the author.