Perhaps an anti - hero could be the main character. They find the Infinity Armor and initially use it for their own self - serving purposes. But as the story progresses, they start to see the bigger picture and become more heroic, using the armor to protect not just themselves but also the different worlds they encounter during the crossover.
One idea could be to have the Infinity Armor cross over with a superhero universe. The wearer of the armor could team up with popular superheroes to fight a common enemy. Maybe they battle an interdimensional threat that only their combined powers can defeat.
Sasuke could also be a great main character in this fanfiction. Given his drive for power, he might be the one to seek out the dragon armor and use it to further his own goals. He could have an interesting journey of learning to control the power of the armor while also dealing with his inner demons.
Harry Potter himself could be the main character in this fanfiction. Imagine if he found the basilisk armor and used it to his advantage against Voldemort. It would add an extra layer of protection and power to his fight against the dark lord.
Wanda Maximoff could also be the main character. After losing Vision, she could be on a rampage. But then she realizes that she needs to control her powers and use them for the greater good. She could be a key player in trying to bring back the vanished ones and fighting any new threats that emerged after Infinity War.
It could be a new vampire character that is created specifically for this crossover. This vampire could be a hybrid of some sort, half - vampire and half - something else, and their unique nature draws the attention of both the vampire world and the characters from 'The Originals'.
Captain America could be the main character. He's always been a leader, and in the aftermath, he could be the one trying to hold the remaining Avengers together. He would be dealing with his own sense of loss and responsibility. He might also be trying to figure out how to prevent future disasters like Infinity War.
Another option is that a new character introduced in the crossover could be the main character. For instance, in a Naruto - Fullmetal Alchemist crossover, an alchemist from the FMA world could stumble upon Naruto's sealed state and make it their mission to free him, while also getting involved in the politics and power struggles of the Naruto world.
A werewolf alpha could be the main character. He enters the 'familier of zero' world and has to navigate the complex magic and politics there. His werewolf instincts could either help or hinder him as he tries to understand this new world and find a way back home.
It could also be a new character created specifically for the fanfiction. This character could be from the world of the crossover and have some special ability or knowledge that helps in the fight against the Heartless. Maybe they have a deep understanding of the magic or technology in their world that can be used to counter the Heartless threat.
The main villain could be Kabuto. He's always been power - hungry. If he got hold of an Infinity Stone, he would use it to try and take over the ninja world. He could use the stone's power to enhance his own medical ninjutsu to the point where he could resurrect and control any dead ninja.