It might be Sam Winchester. Maybe in a different take on the Supernatural world, Castiel has developed a special connection with Sam. Sam is often the more level - headed one, and Castiel could see in him a stable and loving partner. The fanfic could explore how their relationship has grown from just acquaintances to something much deeper, leading to Castiel's decision to propose.
The recipient could be a new character introduced in the fanfic. This character could be a human who has been helping Castiel understand the human world better. They've been on many adventures together, and through these shared experiences, Castiel has fallen in love. This would give the writer a lot of creative freedom to develop a unique relationship and a fresh take on Castiel's romantic side outside of the main characters of the show.
The fanfic could be about a time - travel aspect of Castiel's proposal. He realizes that in order to have a future with his beloved, he needs to go back in time to fix some things. So, he travels through different timelines, experiencing various versions of their relationship. Along the way, he discovers what is truly important in their connection. When he finally comes back to the present, he proposes with a newfound understanding and depth of love, making the proposal a very profound moment that ties together all the different timelines and experiences they've had or could have had.
It could be Sasuke. Naruto has always seen Sasuke as a rival but also someone with great potential. He might be writing to Sasuke as a hero in his own way, someone who has overcome so much darkness. He could be urging Sasuke to fully embrace the path of good.
Perhaps in the fanfic, there is some sort of magic that binds their fates together. Snape might discover this connection and, despite their past differences and his own reserved nature, he decides to propose to Harry as a way to accept this newfound bond. It could also involve Snape trying to make amends for his past actions towards Harry by offering this ultimate commitment.
One possible plot is that Bakugou has been secretly in love with Deku for a long time. He observes Deku's kindness and growth, and it makes his heart flutter. He decides to propose when they are alone one day. He might start by reminiscing about their journey together, from being rivals to becoming something more, and then pop the question.
It could be Umbridge. After all the horrible things she did at Hogwarts, Harry might send a Howler to her when he has the chance. The Howler would be a great way to get back at her for her cruel punishments and her attempts to undermine Dumbledore and the whole Hogwarts community.
Another possibility is one of Naruto's students. He might have taken on some apprentices over the years, and he could pass the torch to the most dedicated and skilled one among them. This student would then carry on Naruto's legacy in the way Naruto taught them, whether it's in protecting the village or promoting peace among the ninja clans.
It could also start with a misunderstanding. Maybe Draco is wrongly accused of something, and Hermione is the only one who believes in his innocence. This act of faith on Hermione's part makes Draco reevaluate his feelings towards her. He starts to seek her out more often, and they begin to have private conversations. These conversations gradually become more personal and intimate. They start to share their dreams and fears. As they become closer, Draco decides he wants to spend the rest of his life with Hermione, leading to the proposal.
The main characters are Bakugou and Deku. Bakugou is often portrayed as a brash and confident character, while Deku is kind - hearted and determined. In this fanfic, their relationship takes a romantic turn with Bakugou being the one to propose.
One possibility is that Naruto would plan an elaborate and very public proposal. Since he is a bit of an attention - grabber, he might do it during a big festival in Konoha. He could gather all their friends and family, make a big speech about how Hinata has always been there for him, through his toughest times, and how he realizes she is the one he wants to spend his life with. Then, he would get down on one knee and present her with a ring.
I'm sorry, I don't know who the author of this fanfic is. There are so many fanfic writers out there, and without more information, it's impossible to determine.