One possible plot could be that they meet in an alternate universe where timelines have merged. Captain Carter and Captain America initially have a bit of a misunderstanding as they come from different versions of heroics. But then they realize they share the same values of justice and freedom. They team up to fight a common enemy that is threatening both their worlds, like a powerful time - traveling villain who wants to rewrite history to suit his evil plans.
Red Skull could be the villain. Since he has a long history with Captain America, in a fanfiction where Captain Carter is involved, he could be resurrected or have found a way to cross over from another dimension. He would see Captain Carter as an interesting new threat and Captain America as an old foe to be finally defeated.
Well, they might start off as a bit cautious around each other. Captain America could be impressed by Superman's superpowers which are on a whole different level compared to his own enhanced human abilities. They could end up teaming up to fight a common enemy, like an alien invasion or a super - villain who is too powerful for either of them alone.
Friendship is a big theme. Despite their differences, the characters usually end up forming strong bonds. They learn from each other and support one another in facing various challenges. Another theme is the exploration of power. Captain America has his super - soldier abilities, and the Hetalia characters have their own skills and influence. How they combine their powers or deal with differences in power is often a part of the story.
Well, one possibility could be that they team up to fight a common enemy. Maybe a powerful villain has emerged that neither of them can take on alone. They start off not trusting each other fully, Wolverine being the lone wolf type and Captain America used to leading his own team. But as they face challenges together, they learn to respect each other's skills and eventually become great allies.
In a 'Harry Potter meets Captain America' fanfiction, perhaps they could team up to fight a common enemy. Maybe Voldemort and Hydra form an alliance, and Harry with his magic and Captain America with his super - soldier skills have to combine their forces. For example, Harry could use his invisibility cloak to help Captain America in a stealth mission against the joined forces of the Death Eaters and Hydra agents.
One idea could be a story where Captain America and Captain Marvel team up to face a new cosmic threat that's headed towards Earth. They have to combine their unique skills, with Captain America's strategic mind and combat experience on the ground, and Captain Marvel's incredible cosmic powers in space battles.
I would suggest 'Unexpected Allies'. This fanfiction takes a different approach as it starts with a misunderstanding between the Hetalia characters and Captain America but then they gradually come to understand and respect each other. It's a great read. Also, 'The United Front' is quite good. It has a complex plot that involves both the political aspects of Hetalia and the heroism of Captain America.
One potential conflict could be their different views on power. Captain America believes in using power responsibly for the good of all, while Nico has powers that are often associated with death and the underworld. This might make Captain America a bit skeptical of Nico at first.