A great plot point would be the exploration of their past traumas. Severus has his childhood issues and his complicated relationship with Lily, and Sirius has his family problems and his time in Azkaban. In the fanfiction, their marriage could be a way for them to heal each other. They could support each other through nightmares and flashbacks. Additionally, a plot point could be their participation in wizarding politics together. As a married couple, they could advocate for new laws or reforms in the wizarding world, like better treatment for werewolves or more inclusive education at Hogwarts.
The conflict within the relationship itself could be a fascinating plot point. They are both strong - willed characters. Severus is often brooding and sarcastic, while Sirius is impulsive and passionate. There could be arguments over small things like who takes care of the household chores or big things like their different views on how to handle dark wizards. But through these conflicts, they learn more about each other and grow closer. Also, a plot point could be their adoption of a magical creature, like a Kneazle or a Hippogriff, and how they raise it together as a couple.
In a fanfiction, perhaps there's a magical accident that causes Severus and Sirius to swap bodies for a while. During this time, they are forced to understand each other on a much deeper level. They see the world through each other's eyes and start to appreciate the other's qualities. This new - found understanding leads to a friendship that blossoms into love, and they get married. It could be a story full of humor, as they adjust to each other's quirks while in the wrong bodies.
I'm not sure if there is such a specific fanfiction. Fanfiction can cover a wide range of themes and ideas, but this one seems rather unusual. You could try searching on popular fanfiction websites like Archive of Our Own or FanFiction.net to see if it exists.
One plot idea could be that they face opposition from the wizarding community due to their past enmity. But they work together to overcome it. For example, they might hold a press conference in Diagon Alley to explain their relationship. Another idea is that Severus, being pregnant, discovers a new potion related to the health of the baby and the magical bond between him and Harry. And they could also go on a journey to find a magical artifact that will protect their unborn child from dark forces.
One interesting plot point could be the reactions of their Pokémon. For example, Pikachu might be a bit jealous at first because Ash is spending so much time with Cynthia. Another plot point could be the cultural differences between Kanto (where Ash is from) and Sinnoh (Cynthia's region). They could incorporate elements from both cultures into the wedding. Also, a plot point could be the challenges they face before the wedding, like an evil team trying to disrupt the event because they fear the power of their combined Pokémon knowledge and skills.
In a fanfiction about their wedding, a plot point could be a mix - up with the wedding dresses. Jessie ordered a specific one but gets a completely different design. She has to decide whether to go with it or try to find the original in a very short time. Another interesting aspect could be the bachelor and bachelorette parties. Maybe something crazy happens at Luke's bachelor party that makes Jessie a bit worried at first, but then she realizes it was all in good fun. And there could be a moment during the ceremony where the officiant forgets the words, leading to a humorous and heartwarming moment.
Maybe there's a plot where they have a secret wedding ceremony in a magical place, like a hidden grove in the forest that only Merlin knows about. And then they have to deal with the implications of being married while still maintaining their public personas in Camelot.
One interesting plot point could be the reactions of their friends and allies. How would the members of the Occult Research Club respond? Another could be the challenges they face in planning a wedding that suits both their demon and human - influenced cultures. For example, they might have to deal with traditional demon wedding customs that Issei is not familiar with.
One common theme is the struggle between good and evil. Since Sirius is typically seen as a 'good' character and Bellatrix as 'evil', the fanfictions often explore how their relationship might change their natures. Another theme is family loyalty and betrayal. As members of the Black family, their union can bring out different reactions from family members.
One plot idea could be that Severus and Lily, after getting married, have to deal with the secrets Severus has from his Death Eater past. Another could be them raising Harry together, but facing opposition from the wizarding world due to Severus' former associations. A third idea might be that they discover a new magical threat and use their combined knowledge to fight it, all while strengthening their relationship.
One interesting plot point could be how Harry and Severus initially react to the marriage contract. Maybe Harry is shocked and resistant at first, while Severus is more resigned to it due to his sense of duty.