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.
Hermione is a very common character. Given her intelligence and magical skills, she often plays a crucial role in saving Harry. Ron is also often involved as Harry's loyal friend. Dumbledore, even in fanfiction, is a character who might show up to save Harry or at least guide the others in the rescue.
One common theme is friendship. Often, it's Naruto's friends who come to his rescue, showing the strong bonds in the Naruto world. Another theme could be redemption. Maybe the one who saves Naruto is someone who was once his enemy but now has a change of heart.
One popular 'naruto gets saved fanfic' could be 'A Helping Hand'. In this fanfic, Naruto is saved from a dangerous situation by an unexpected ally. It has an interesting plot that shows how Naruto's life could take a different turn with this intervention.
Well, in certain 'harry gets saved fanfiction', Hermione might use her vast knowledge of magic in a completely new and unexpected way to save Harry. She could combine different spells and charms that no one has thought of before. Also, there could be fanfictions where Neville, having grown in confidence and power, is the one who saves Harry from a life - threatening encounter with a dark wizard.
One possible plot for this fanfiction could be that Deku is facing a powerful and unknown enemy that has the ability to nullify his powers. He's in a desperate situation when Kurogiri shows up. Kurogiri, for reasons unknown to Deku at first, uses his portal - creating abilities to rescue Deku. It could turn out that Kurogiri has a connection to Deku's past that he's not aware of. Maybe Kurogiri was once influenced by someone who saw the same kind of potential in Deku that he himself had when he was younger, and that's why he saved him.
Well, one possibility could be that Naruto is in a really tough battle, perhaps against a group of powerful enemies. Anko could show up just in time. She might use her snake-based jutsu to fend off the attackers. For example, she could summon snakes to entangle the enemies, giving Naruto a chance to catch his breath and regain his strength.
Yes, there are quite a few. You can start by looking on popular fanfic platforms like Archive of Our Own. There, you might find some well - written stories about Reiner and Mikasa. Some focus on their complex relationship considering their different allegiances in the original story, which makes for really interesting reading.