There's also 'Jujutsu and Bungo in an Alternate Realm'. This fanfic delves deep into how the powers from both universes would interact. It shows the characters trying to understand each other's abilities and form alliances. It's a long read but definitely worth it for fans of both series.
One interesting Spy x Family reaction fanfiction could be where the Forgers react to Anya getting a perfect score on a test. It's fun to see Loid's shock and Yor's pride. Another might be their reactions to a new neighbor who seems a bit suspicious. There are also fanfictions where they react to Anya's first school play.
The idea of a 'dirty fanfic' in this context is not something to be encouraged. Instead, if one is interested in the sans x chara relationship, they could write a story about how they might team up to solve a mystery in the game world. This would be a much more positive and family - friendly take on their relationship.
There are fanfictions where the main characters react to different versions of their powers. For example, Gon and Killua reacting to a more powerful or darker version of their nen abilities. This can lead to interesting character development as they grapple with what they could become. It also allows the author to explore different power - up scenarios in a unique way.