Perhaps the self - insert character is a scientist who accidentally fuses with the Reality Stone. This gives them the power to change reality at will. They must learn to control this power while also dealing with the Avengers who are initially suspicious of this new entity in their world. They could use their power to recreate battle scenarios to train the Avengers better.
A plot could revolve around a self - insert being transported back in time and having to gather the Infinity Stones before Thanos does. Along the way, they form unexpected alliances with the Avengers from different timelines. They might start with finding the Space Stone and use it to travel between planets in search of the others.
Another way is if the self - insert character is from another dimension and gets stranded in the Avengers' world. They have an Infinity Stone that is malfunctioning, and the Avengers offer to help. In the process, they learn about each other's cultures and powers. The self - insert could teach the Avengers some unique combat techniques they have from their own dimension.
One idea could be exploring the backstories of the minor characters in more depth. For example, showing how the Wakandan warriors trained and what their lives were like before the big battle. Another could be a what - if scenario where Thanos actually had a change of heart at the last minute. Maybe he saw the beauty in the universe he was about to destroy and decided to stop.
Another plot could focus on the relationship between Thanos and his adopted daughters in a more in - depth way in the context of the Infinity Wars. Maybe there was a time when they were planning a rebellion against him before he even started his quest for the Stones. This could add a whole new layer of backstory and drama to the events of the movie.
A great plot idea is that the self - insert vampire is actually a lost member of a forgotten clan in the Naruto world that has vampire - like traits. They are initially feared by the other characters but gradually prove themselves to be an ally. Or perhaps the vampire is from another dimension entirely and is accidentally pulled into the Naruto universe. They then use their unique abilities to help solve a major crisis in the world, like a war or an impending doom caused by a powerful enemy.
One idea could be that Hela, as the self - inserted character, tries to reform the Avengers from within. She could use her unique powers and knowledge of Asgardian lore to help them face a new, powerful enemy. Maybe she starts off as a suspicious figure but gradually gains their trust through her heroic deeds.
Well, in an 'Aint no grave avengers fanfic', it might be about an Avenger who has been presumed dead but actually isn't. They come back with a vengeance, having faced some sort of otherworldly experience in the 'grave'. Their return shakes up the team dynamics as they have changed during their time away. They might bring back knowledge or powers that are both a boon and a challenge to the rest of the Avengers.
One plot idea could be that your OC self - insert discovers a hidden power within themselves that is connected to an ancient prophecy in the High School DxD universe. This power makes them a target for various factions, both good and bad. Another idea is that they accidentally stumble upon a secret alliance between different species that is planning something big, and they have to decide whether to expose it or get involved.
One idea could be that your self - inserted character is a long - lost Saiyan who has unique powers. For example, the ability to control time on a small scale during battles, which would add a new dynamic to the fight scenes in the Dragon Ball Hero universe. Another plot could involve your character being from a different dimension and having to adapt to the rules and power levels of the Dragon Ball Hero world while also trying to find a way back home.
One plot idea could be that your self - insert character has a special ability to sense ancient power, and teams up with Robin to find a hidden Poneglyph that is said to hold the key to a great treasure. Another is that your character is from a lost civilization and has some knowledge that Robin needs to complete her research on a particular historical event. A third idea is that your self - insert gets lost in a mysterious library on an island, and Robin has to find and rescue them while also uncovering some long - lost knowledge within the library.
Perhaps a plot where the Futa character has a special power related to the sea, and Female Luffy, with her dream of becoming Pirate King, needs that power to access a hidden island full of treasures. Their journey together is filled with adventures, like battling new sea monsters and dealing with other pirate crews who also want the power.