How about a young, inexperienced wizard from a different magical realm? This wizard stumbles upon the knowledge of the Infinity Stones and decides to help the Avengers. They bring with them a different style of magic that could be used in creative ways against Thanos and his minions.
A character from a long - forgotten Asgardian legend could show up. This character could have powers that are related to the Infinity Stones in some way, like being able to sense when they are being misused. They could join the Avengers in their fight against Thanos.
The postman could be an unexpected hero. Imagine he's been delivering mail all over the city and has unknowingly witnessed some strange activities related to Thanos' minions. One day, he stumbles upon a piece of alien technology that gives him the power to disrupt Thanos' communication network, which turns out to be a vital part of the Avengers' victory.
Gandalf from The Lord of the Rings. Imagine him showing up in the Marvel universe, using his magic to help the Avengers. His wisdom and powerful spells could be a great addition, especially in a battle against a powerful foe like Thanos. He could also offer different perspectives on heroism and sacrifice.
A young Twi'lek mechanic who is forced to work for the Infinite Empire but secretly sabotages their ships. She could be really smart and resourceful, using her skills to help the resistance.
Howard the Duck. It would be really unexpected. He could bring some comical relief but also have some hidden knowledge about the disks that could be crucial. For instance, he might know of a weakness in a powerful enemy's disk - based powers.
Some fanfictions could be crossovers. Picture a scenario where the Avengers from the Infinity Wars team up with characters from other Marvel universes, like the X - Men. They could combine their powers and strategies to face the threat of Thanos in a completely new and exciting way.
Sherlock Holmes could be an unexpected character in Pinocchio crossover fanfiction. Holmes could use his deductive skills to figure out why Pinocchio's nose grows when he lies and might even try to find a way to control it. Another one could be Alice from Wonderland. Her strange adventures in Wonderland and Pinocchio's own adventures could combine in interesting ways, like exploring a new and magical land together.
One idea could be that in the fanfiction, instead of Thanos getting all the Infinity Stones, the Avengers manage to steal some of them first. They could then use the power of the stones they have to create alternate versions of themselves to fight on different fronts against Thanos' forces.
Hawkeye. He's often overlooked but could be really interesting in a fanfic. He could have a solo mission during the events of Infinity Wars where he discovers a secret about one of the Infinity Stones that no one else knows. Maybe he has to use his archery skills in a completely new and creative way to protect this knowledge from falling into the wrong hands.
First, study the characters. Know their personalities, powers, and relationships. For instance, Tony Stark's intelligence and sarcasm are key aspects. Then, think of a unique plot. It could be something that changes the outcome of a major battle.
There could also be a character like Dara, a smuggler who gets caught in the middle of the New Sith Wars. She doesn't care about the Jedi - Sith conflict at first, but as the war escalates, she realizes she has to choose a side to protect her own interests. She could be a morally ambiguous character, willing to do whatever it takes to survive.