Eren's death in the manga was a powerful moment. It was brought about by a combination of his own decisions, the opposition he faced, and the overall plot developments. It left a lasting impression on readers.
Armin could be one of the main characters. He might be the one who offers the most logical advice to Eren during his pregnancy. For example, he could help Eren figure out how to deal with the medical aspects if the world in the fanfiction has different rules for male pregnancy. Another character could be Mikasa. She would probably be very protective of Eren, just like she always is. She might be the one to stand up for Eren against any discrimination or threats he faces because of his pregnancy.
Eren managed to beat Annie through a combination of his determination, newfound powers, and strategic thinking. He exploited her weaknesses and caught her off guard.