A scientist could be a main character. They would be interested in studying Captain Olimar, trying to understand his biology and the technology he uses. Maybe a child as well. Children are often curious and open - minded, so they could form a quick bond with Olimar, teaching him about simple human things like toys and snacks.
Well, the fanfiction could start with Captain Olimar being teleported to a human city by accident. He wanders around, confused, and catches the attention of a young journalist. The journalist decides to help Olimar and document his experience. As they interact with more people, they encounter different reactions - some are scared, some are curious. Olimar starts to learn about human emotions, which are very different from what he's used to in his own species. Along the way, he also helps the humans solve a local environmental problem using his knowledge from his own planet, and in return, the humans help him find a way back home.
A mysterious figure from the captain's past could be a key character. Maybe this person was involved in the events that led to the promise in the first place. They could reappear and try to either help or hinder the captain in keeping the promise. Also, a young crew member who looks up to the captain could be important. Their naivete and admiration could add an interesting dynamic as they try to understand the weight of the captain's promise.
Maybe a group of bounty hunters hired by the remaining Imperial forces. They see Luke as a big threat and Captain Rex as an annoyance. They keep hunting them down throughout the story, creating all sorts of dangerous situations for our heroes.
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.
Urahara Kisuke would be a great addition. His mysterious nature and vast knowledge could either help or hinder Naruto. Yoruichi could also be a main character. Her cat - like abilities and combat skills would make for some exciting interactions with Naruto. She could teach Naruto some new moves or they could team up against a common enemy.
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.
Adventure is also common. They are known for their adventures in the game, and in fanfics, this is often expanded upon. They might go on new quests or explore uncharted areas together. This allows for a lot of creativity in terms of the settings and obstacles they face. For example, in some fanfics, they might encounter strange new Pikmin species or deal with dangerous new enemies. It gives a chance to further develop their characters as they react to these new situations.
A good antagonist could be a rogue S.H.I.E.L.D. agent who has his own plans for power. He might see Captasha and Black Widow as threats to his scheme. This agent could have access to some high - tech weapons and a group of loyal but misled followers.
Well, obviously Captain Darko is a main character. He's the central figure around which the stories revolve. There might also be his trusty first mate, someone who has been with him through thick and thin. This character could provide support and sometimes comic relief.
Obviously, Percy is the main character as he is the captain. But depending on the type of fanfiction, there could be other important characters. For example, if he's a sports captain, his teammates would be significant. They could have different personalities and skills that interact with Percy's leadership. If he's a ship captain, his crewmates, like the navigator or the first mate, would play important roles in the story.