It depends on personal preferences. Some people might think Tokyo Ghoul is better for its original story and character development. Others might prefer Tokyo Ghoul:Re for its continuation and new plot twists.
The biggest distinction is that Tokyo Ghoul:re builds upon the foundation of Tokyo Ghoul and expands the world and lore. The character arcs evolve differently, and there are new challenges and conflicts. Both manga have their unique charm though.
It depends on your taste. Some people love the depth of the plot and the evolution of characters in Tokyo Ghoul:re manga, while others might find it a bit too dark or complex. But overall, it has its merits.
A possible fanfiction could focus on their past. Maybe they first met when Nishiki was still struggling with his own identity as a Ghoul. Kimi was the one who showed him kindness and acceptance. As time goes by, Nishiki starts to develop feelings for Kimi, but he's afraid to admit it because of the dangerous world they live in. The story could explore how he overcomes this fear.
First, their personalities. Nishiki is often seen as a bit of a rough character, while Kimi is more gentle. So showing how these different personalities interact is important. Second, the Ghoul world setting. You can include things like the struggle for food, the danger from other Ghouls and investigators. Third, their relationship development. Maybe start with a simple acquaintance and gradually build up to deeper feelings.
The theme of forbidden love often comes up as well. In the Tokyo Ghoul universe, the relationship between a ghoul and a human is generally not accepted. So, in Nishiki x Reader fanfictions, there's often the tension of this forbidden love, with the two having to hide their relationship from others, whether it's other ghouls or the CCG.
I think there are a specific number of volumes, but I'm not sure exactly. You might want to check reliable manga databases or fan forums for the precise count.