Develop the side characters well. Have them react to Issei's situation. They can offer support, or in some cases, make the situation worse. For instance, a jealous character might try to keep Rias and Issei apart, adding more drama to the 'rias breaks issei hurt' scenario.
One possible 'rias breaks issei hurt' fanfiction could be about a misunderstanding. Maybe Rias misinterprets something Issei does and it leads to a break in their relationship, causing Issei emotional pain. For example, Issei might be trying to protect Rias in a secretive way and she mistakes it for betrayal.
You can start by searching on popular fanfiction websites like FanFiction.net or Archive of Our Own. Just type in 'rias and issei kiss' in the search bar, and you'll get a list of relevant stories.
One way is to add a common enemy. By introducing a powerful foe that both Naruto and Rias have to face together, they can gradually overcome their initial hatred. For example, a new, extremely powerful demon or a group of rogue ninjas that threaten both their worlds.
You can start by looking on popular fanfiction websites such as Archive of Our Own or FanFiction.net. Use the search bar and type in 'rias and issei pregnant' to get a list of relevant stories.
Issei would most likely become more protective. He'd be constantly by Rias' side, making sure she was comfortable and safe. He might also start training even harder to ensure he could defend his family when the baby arrives. His character would grow in the sense that he'd become more selfless and family - oriented, thinking about the future of his family rather than just his own growth as a fighter.
Involving other characters in an interesting way can make it more engaging. Maybe there's a character who is trying to understand what the baby is experiencing during hypnosis. They could be an older sibling or a parent. Their reactions and attempts to communicate with the baby can add depth to the story. Also, creating a sense of mystery around the hypnosis itself, like not revealing all at once why the baby was hypnotized or what the full extent of its effects are, can keep the readers hooked.
Adding a lot of vivid descriptions. For example, if the story is set in a forest, describe the lush greenery, the sounds of the animals, and the smell of the damp earth. This makes the setting more real for the readers.
Adding vivid descriptions of the emotions involved can make it more engaging. For example, describe the mother's fear, the father's anxiety.
To make a 'Naruto is heartbroken' fanfiction more engaging, focus on his emotions deeply. Describe how his heartbreak affects his daily life, like he might stop eating or training. Also, add some flashbacks to happier times to contrast with his current state.
By developing strong characters. Make Gamzee's character multi - dimensional, with his own hopes, fears and quirks. Also, create interesting antagonists if there are any involved in the abuse situation.