It could be really exciting. Imagine Spider - Man swinging into the Justice League's world. He might team up with Superman to take on a powerful new villain. Maybe they would have some initial misunderstandings due to their different ways of operating, but then quickly realize they can complement each other's skills.
Well, it could be amazing. Picture Spider - Man's acrobatic fighting style combined with the superpowers of the Justice League members. Maybe he teams up with Superman to save a city from a huge threat. Their different personalities would create interesting interactions, like Spider - Man's quippy nature against Superman's more serious and noble demeanor.
Maybe the plot could involve a device created by Lex Luthor that accidentally merges the two universes. Spider - Man then teams up with the Justice League to find a way to separate the universes again. During this time, he has to deal with the different personalities and powers of the League members. He might form a special bond with Batman, as they both rely on their wits and gadgets.
Spider - Man could fit in as the tech - savvy member. His knowledge of creating his own web - shooters and other gadgets could be useful for the Justice League. He could upgrade their equipment or come up with new ways to fight villains.
Well, in such a fanfiction, Spider Man could show off his acrobatic skills to the Justice League members. Maybe he'd teach Flash a thing or two about speed and agility in a friendly competition. And he could collaborate with Batman on some detective work, combining his street - smart knowledge with Batman's high - tech resources.
There could be a storyline where an evil force has brainwashed some of the Justice League members, and Spider - Man has to figure out a way to break the spell. Along the way, he teams up with the unaffected members like Wonder Woman. This would show his intelligence and resourcefulness. Also, a story where Spider - Man accidentally ends up in the Justice League's universe and they have to work together to get him back home while also dealing with threats in their world.
It could be really exciting. Picture Naruto's ninjutsu skills combined with the superpowers of the Justice League members. For example, Naruto's shadow clone jutsu could be used in a really creative way during a battle against a common enemy that the Justice League and Naruto face. Maybe he could create clones to confuse villains like Lex Luthor while Superman uses his heat vision to take them down.
A cool plot could be that the Justice League discovers a strange energy signature that turns out to be Iron Man's arc reactor. They track it down and find Iron Man. Iron Man is initially out of his depth in this new world of superpowers. But he uses his wits and technology. For example, he could upgrade the Flash's suit to make him even faster, and in return, the Flash helps him understand the new world he's in. Together, they all face off against Darkseid who has plans to invade both universes.
Spider - Man's agility and web - slinging could be combined with Superman's flight. For example, Spider - Man could use his webs to attach to Superman while he's flying, allowing them to reach places faster. Or Batman could use Spider - Man's web - shooters in his utility belt for a new way to immobilize enemies. Flash and Spider - Man could work together to create a super - fast web - trap for villains.
It could work if there was a common enemy that forced them together. In the fanfiction, this enemy might be so powerful that they need to combine forces. And as for the harem aspect, it could be that Spider - Man's charm and his outsider - status make him appealing to some of the Justice League members. They might see him as something new and exciting, different from their usual comrades. So, they gradually develop feelings for him.
I think Spider - Man and Flash would make for a really fun pair. They both have a sense of speed in different ways. Flash has his super - speed, while Spider - Man can move quickly through the city with his webs. They could have races, and Flash could probably teach Spider - Man how to move even faster. Also, their personalities are quite friendly, so they would likely get along well and have some humorous exchanges in the fanfiction.