It's hard to say. Superman has incredible strength, speed, and heat vision. But Thor has the power of lightning and is a god. Hulk's brute strength is also a force to be reckoned with. In a fanfiction, it could depend on the situation and how the writer wants to portray their power levels.
In such a fanfiction, there could be epic battles. The superpowers of both groups would clash. For example, Superman's strength against Thor's might. It would be a test of different ideologies too. The Avengers might be more about protecting Earth from all threats, while the Justice League has a broader view including intergalactic threats.
Well, in an 'avengers meets justice league fanfic', a lot of character interactions are possible. Thor could share his knowledge of Asgardian magic with Wonder Woman who has her own set of magical items. Captain America and Aquaman might have a conversation about leadership and protecting their respective worlds. Also, the Scarlet Witch could use her powers to create illusions that would trick some of the Justice League members.
I think Superman and Thor would be a very powerful pairing. Superman has his super strength, speed, flight, and heat vision, while Thor has his godly strength, control over lightning, and his enchanted hammer. Together, they could take on almost any foe. They could fly into space and deal with threats that are far away from Earth.
It's hard to say for sure, but Superman could be a top contender. His super strength, speed, and flight, along with his heat vision and other abilities, make him extremely powerful. He can withstand a lot of damage and dish it out just as well.
In my opinion, it could be Scarlet Witch. Her reality - warping powers are extremely potent. In a crossover, she could potentially change the very fabric of the combined universes. If she were to go rogue or be influenced by an evil force, she could cause chaos on a scale that would be very difficult for the combined might of the Justice League and the Avengers to handle. Even Superman's invulnerability might not protect him from her reality - warping abilities. She could simply make him mortal or take away his powers altogether.
In such a fanfiction, there might be a big threat that requires both teams to combine their skills. The Flash could race with Kid Flash and share some speed - related tips. And Wonder Woman could train the female members of Young Justice in combat.
Another possibility is a post - Endgame fanfiction. After the events of Endgame, the Avengers discover a new threat that has ties to the Justice League's world. Maybe a powerful villain from the DC universe has somehow been awakened by the events in the Marvel universe. So they have to reach out to the Justice League for help and together they figure out a way to stop this new threat.
Yes, there are many good ones. You can find them on fanfiction websites. Some focus on epic crossovers between the Justice League and the Avengers, with amazing battles and team - ups.
Brucie in this fanfiction could be someone who has been living on the fringes of society. He might be a street - smart person who has a deep understanding of the underbelly of the city. When the Justice League meets him, they realize that his knowledge can be useful in their fight against crime. He could also be a bit of an outcast, which makes his relationship with the super - heroes all the more interesting.
The Phantom Troupe could form an uneasy alliance with some of the more morally grey members of the Justice League, like Catwoman or Lobo. They might team up to take down a common enemy that the League can't deal with using their usual methods because of legal or moral constraints.