One common theme is teamwork. Since they are both spider - powered heroes, a lot of fanfics focus on how they work together against villains. For example, they might combine their unique abilities to take down a powerful foe.
One possible plot could be that Spider - Man, in his adventures, somehow crosses dimensions and ends up in the Sailor Moon universe. He meets Sailor Moon and they initially have misunderstandings due to their different ways of fighting crime. But as they get to know each other, they start to fall in love. Eventually, they decide to get married, uniting their two worlds in a way. Their friends from both sides would have to adjust to this new situation, and they might face some villains who are against their union.
It all starts when Peter Parker gets bitten by a radioactive spider. This gives him amazing powers like super strength, the ability to climb walls, and a 'spider sense'.
Spider - Man's physical strength, agility, and his spider - sense could work well with Sailor Moon's magical attacks. For example, Spider - Man could use his speed to dodge attacks and get close to the enemy, while Sailor Moon could then use her magic to finish the foe off. His spider - webs could also be used to hold enemies in place while she casts her spells.
I also like 'The Moon and the Parker'. This fanfic focuses on the more personal side of their relationship. It explores how they deal with their dual identities outside of being superheroes. It has a lot of great dialogue that really brings out the personalities of both Cindy Moon and Peter Parker.
One idea could be a story where Miles Morales has to team up with an alternate version of himself from a dimension where he never became Spider - Man. They have to overcome their differences and learn from each other to stop a new villain that is threatening all the spider - verse.