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.
Love and sacrifice are common themes. Often, Reiner has to sacrifice something for the sake of love in these fanfictions. It could be his position in the Legends world or his own safety.
A popular one is 'Reiner's Heart of Gold in Love and Legends'. It shows a different side of Reiner, a more vulnerable and loving side. The story has a great plot that unfolds as Reiner navigates the challenges of the Legends world while falling in love. It has detailed descriptions of the settings and emotions that really draw you in.
The exploration of new powers and abilities is also a common theme. As an OC Titan shifter, the author can come up with unique powers that the character has. This can lead to interesting plotlines, like how the character discovers and masters their powers. And then there's the theme of mystery. The origin of the OC's Titan - shifting ability might be a mystery that is slowly unraveled throughout the story.
The world of Attack on Titan would experience a major transformation with the introduction of Pokemon. Socially, the relationship between people would change as they bond over their Pokemon. There could be Pokemon centers and Pokemarts within the walls. Economically, a new industry would emerge around Pokemon. In terms of combat, as I mentioned before, different types of Pokemon would bring new tactics. Fire - type Pokemon could set the Titans on fire, ice - type Pokemon could freeze them, and so on. Also, the idea of Pokemon evolution could be incorporated into the story. Maybe the more a trainer and their Pokemon fight Titans together, the stronger their bond and the more evolved their Pokemon becomes, making them even more effective against the Titans.