Perhaps it's a wish gone wrong. She makes a desperate wish for a change and gets this unexpected transformation. Now she has to navigate life in a completely different way.
It could be a scientific experiment gone haywire. Some crazy scientist's concoction transforms her gender. And then there's all the drama of her dealing with the new body and society's reactions.
One possible scenario is that they are friends just having a good time, like shopping together or going to a concert. Another could be that they are colleagues discussing work - related matters outside the office. And of course, they could be a couple on a date, enjoying each other's company in public places.
It could be them recovering from an illness or injury. Maybe they just had surgery and are now resting.
Another theme might be identity. His identity as a man is completely shaken when he turns into a pregnant woman. He has to re - evaluate who he is, his gender role, and how society views him. It could be about breaking gender norms and exploring the fluidity of identity.
Maybe they're having a fight about how to spend money or where to go on vacation. Or it could be a work-related disagreement.
Maybe the man did something wrong and the woman is angry about it. Like forgetting an important event or making a big mistake at work.
Another scenario could be that Iron Man is poisoned or infected with a deadly alien virus. He tries to fight it off but eventually succumbs to it. His friends in the Avengers try their best to save him but are unable to. This would be a really tragic way for him to go, especially considering how much he has done for the team and the world.
Perhaps he succumbs to an old wound or illness that has been haunting him throughout the story. In fanfiction, the author might play with the idea of his age finally catching up to him, and he dies a peaceful death in his sleep or while reminiscing about his life.
One possible scenario could be that Spider Man sacrifices himself to save the city from a powerful and unstoppable villain. He might use all his strength and powers to defeat the enemy, but in the process, his body gives out due to the extreme exertion.
One possible scenario could be in a battle against a powerful new villain. Maybe this villain has some sort of weapon that can penetrate her defenses, like a magic blade. Another could be that she's caught in a trap set by her enemies while trying to protect an innocent. In some fanfictions, she might also get hurt because she's overexerting herself to save a large number of people all at once.
Maybe it takes place in an alternate universe where the world is a bit different. Batman is more open with his emotions. He takes Wonder Woman to a special place, like an ancient, hidden garden. There, he tells her how much she means to him, not just as a teammate but as someone he has romantic feelings for. He might talk about how her strength and beauty have always attracted him, and how he has been trying to hide these feelings for a long time but can't anymore.