Well, in some fanfictions, a heartwarming scene is when Bumblebee repairs something precious for the reader. Maybe it's a broken heirloom or a childhood toy. He uses his mechanical skills to fix it up, and the look of joy on the reader's face makes it a really tender moment. Also, when Bumblebee first meets the reader and he's a bit clumsy but in an endearing way. He might accidentally knock something over, and then tries to make up for it in a cute way like bringing the reader a little shiny object he found.
The plot is also crucial. It could involve the reader and Bumblebee teaming up to save the day, or perhaps the reader getting caught up in the Transformers' inter - galactic conflicts. A good plot will have a clear beginning, middle, and end, with enough twists and turns to keep the reader engaged. Additionally, the relationship between the reader and Bumblebee should develop gradually and realistically. It could start from a place of curiosity and grow into a deep friendship or something more, but it should be believable.
I would also suggest looking at some of the official Transformers fan forums. Members there often discuss and share their own fan - created works. They might have a section dedicated to 'Bumblebee x Reader' fics that are family - friendly. These fics could be about the reader getting to know Bumblebee's unique personality, or the two of them forming a special bond based on mutual respect and trust.
When writing this kind of fanfiction, focus on the emotions. Show how the reader feels about Bumblebee, whether it's admiration, fear at first (because he is a giant robot), and then growing trust. For Bumblebee, show his attempts to connect with the reader on a deeper level. Include action scenes too, like battles where Bumblebee is fighting to keep the reader safe. And don't forget about the setting. A post - apocalyptic world or a hidden Autobot base can add a lot of atmosphere to the story.
A fanfiction could revolve around the idea that there is a prophecy involving Bumblebee, Grimlock and this mysterious X. Bumblebee is seen as the key to finding X, and Grimlock is tasked with protecting Bumblebee. Along the way, they face many challenges. Grimlock's dinosaur form might get them out of some tough situations by brute force, while Bumblebee uses his intelligence to solve puzzles. As they get closer to finding X, they learn that X is not what they expected and it has the potential to change the fate of all Transformers.
One common theme is friendship. Despite being on different sides in the original Transformers lore, in fanfiction they might develop an unexpected friendship. Another theme could be adventure, where they team up to face a common threat.
One possible plot could be that the male reader discovers Bumblebee in a junkyard. Bumblebee is damaged and the reader, who has some mechanical skills, decides to help repair him. As they work together, a bond forms between them. They end up going on adventures, facing off against other Decepticons who are causing trouble in the city.
The plot could be that Blitzwing and Bumblebee are both captured by a new powerful enemy. They are put in a gladiator - like arena where they have to fight other captured Transformers. At first, they are at odds with each other, but they soon realize that they need to work together to escape. They use their unique abilities, like Blitzwing's ability to transform into different forms and Bumblebee's agility, to fight their way out.
One great Bumblebee x Reader fanfiction is 'Bumblebee's Protective Embrace'. It shows Bumblebee being very protective of the reader in the middle of a battle on Cybertron. The author does a wonderful job of describing the action scenes and the emotional connection between Bumblebee and the reader.
A possible heartwarming scene could be Loki sharing his Asgardian stories with the unborn baby through the reader's belly. He'd talk softly about the grandeur of Asgard, its golden halls and brave warriors. This not only creates a connection between him and the baby but also shows the reader how much he values their future family. Also, when the reader is feeling self - conscious about her changing body, Loki would shower her with compliments and reassure her that she is the most beautiful being he has ever seen.
She could be a spy within the Decepticon ranks. Pretending to be on their side while secretly feeding information back to the Autobots. Her small size and ability to transform into a less - conspicuous vehicle would make her perfect for this role.