Some fanfics also focus on the psychological aspect of their relationship. Reiner and Bertholdt both have complex personalities and backstories. Fanfics might explore how they understand and support each other on a psychological level, perhaps dealing with their inner demons together.
Fanfiction.net is also a great place to look. There are many 'Attack on Titan' fanfics there. Just type in 'Reiner x Bertholdt' in the search bar and browse through the results. However, the quality may vary, so you might need to read a few to find the really good ones.
For Eren, he could start off as a bit of a rebel in high school. Maybe he's always getting into trouble for questioning the school's rules. But as he spends more time with Mikasa, he starts to see the importance of following some rules and also becomes more empathetic. Mikasa, on the other hand, can be developed as the quiet, studious type who slowly comes out of her shell because of Eren's influence.
One plot idea could be Eren and Mikasa being in a high school drama club together. They get cast as the lead couple in a play, which makes their real - life feelings for each other start to surface. As they rehearse, they face challenges like jealousy from other classmates who also have a crush on them.
It's fairly common. Fans often look for more content related to their favorite shows. Since 'Attack on Titan' has a large and passionate fan base, there are a lot of fanfics available. 'Aot' fans might watch (read) these fanfics to explore different scenarios that the official story didn't cover. For example, they might want to see how a certain character would react in a different situation or what would happen if a major plot event had a different outcome.
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.
There could be various fanfictions. Maybe some that focus on alternative endings, like ones where certain characters survive who didn't in the original. For example, a fanfiction where Erwin survives and leads the Scouts in a different way.
It could mean that a person who is a fan of 'Attack on Titan' (AOT) is reading or watching fan - created fictional works related to AOT. Maybe they are interested in seeing different interpretations or storylines that fans have come up with.