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 possible fanfiction could be about War and Uriel teaming up to face a new, unknown threat in the Darksiders universe. Maybe a powerful demon that has emerged from a previously sealed realm, and they have to combine their unique abilities to defeat it.
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.
In general, the Evangelion mangas are considered good. They expand on the anime's universe, providing more depth and character development. However, it depends on personal preferences as some might find the style or pacing not to their liking.