Well, they might have a deep conversation about their different views on the world. Eren could try to convince Zeke of his more radical approach to saving the Eldians, while Zeke might push back with his own ideas of euthanasia for the Eldians. It could be a very tense encounter.
They could start to plan a strategy to deal with the Marleyans together. Since both have their own experiences with Marley, they might come up with a complex plan involving Titan powers and political manipulation. For example, using Zeke's position as a Marleyan weapon and Eren's influence within Paradis.
One element is the lyrics. If the lyrics are about dark topics like death, ghosts, or curses, it can be scary. For example, lyrics that describe a spooky encounter with a supernatural being. Another is the tone of the rap. A slow, deep, and menacing tone can add to the scariness. Also, the setting where the rap takes place. If it's in an abandoned place like an old factory or a haunted house, it gives a creepy vibe.
One interesting grisha fanfiction could be about a grisha with a unique power that wasn't explored in the original series. For example, a grisha who can manipulate time on a small scale. This could lead to all sorts of adventures and challenges as they try to keep their power a secret while also using it to help their friends.
Love is also a common theme. Sometimes it's a love between two Grisha, and other times it's a love that crosses the boundary between Grisha and non - Grisha. For instance, a story might show how a Grisha falls in love with a normal person and has to deal with the differences. Additionally, the theme of survival is prominent. Given that Grisha are often in a dangerous world, they have to fight for their survival, both from physical threats and from those who fear their power.
Well, here's one. In a small town, there was a rap battle. A young rapper named Jay took part. He rapped some really bold lyrics. But after the battle, he started having nightmares. In his dreams, he was in a dark room and a shadowy figure was rapping at him, criticizing his skills. Every night the dream got worse, and the figure got closer. One day, he decided to stop rapping, and the nightmares finally ceased.
You can start by looking on popular fanfiction websites such as Archive of Our Own. There, you can simply search for 'grisha' and a whole list of fanfictions will come up. You can then sort them by popularity, rating or length to find ones that might interest you.