One of the best SNK fanfics could be those that focus on the backstories of the characters. For example, a fanfic that delves deep into Levi's past before he joined the Survey Corps. It might explore his life in the underground and how he developed his combat skills. Another great one could be about Eren's internal struggle in a more detailed and alternative way than in the original story. There are also fanfics that explore different relationships between the characters, like a friendship - centric story between Armin and Mikasa that shows their bond growing in unique circumstances.
One characteristic could be its unique art style. SNK is known for its distinct visual aesthetics, which are likely to be carried over into the visual novel. Another might be its storytelling. It could potentially have complex and engaging plots, perhaps with elements of mystery or action.
Reiner did die in the manga. His death was a result of a series of events and conflicts that had been building up throughout the storyline. It had a major impact on the plot and the other characters.
One common theme is power struggle. When characters from different universes meet in a crossover, they often have different levels of power. For example, in a crossover with a superhero universe, the SNK characters might have to deal with the much greater powers of the superheroes, leading to a power struggle.
You can start by looking on popular fanfic websites such as Archive of Our Own (AO3). It has a large collection of SNK fanfics and a good tagging system that allows you to filter for the best - rated ones. Another option is Fanfiction.net, which also has a decent number of SNK fan works. You can also join SNK - related fan communities on social media platforms like Reddit or Tumblr. Members often share and recommend their favorite fanfics.
The mystery aspect. In a 'cryptic lover' SNK fanfic, the relationship between the characters is often veiled in secrecy. It makes readers curious and eager to figure out how the relationship will unfold.