In some cases, an antagonist could be a dark force or an ancient evil that has been awakened. This entity might target Inuyasha specifically because of his unique position as a half - demon. Sesshomaru then has to use all his skills and strength to fend off this threat and protect Inuyasha from this unknown and powerful foe.
One common plot element could be the growth of the relationship between Sesshomaru and Inuyasha. For example, at first Sesshomaru might protect Inuyasha out of a sense of duty or family obligation. Another element could be the challenges they face together. Enemies from their past might resurface, forcing Sesshomaru to use his powerful abilities to keep Inuyasha safe. Also, there could be a development in Inuyasha's character as he learns to trust Sesshomaru more and more.
One popular 'little inuyasha and sesshomaru fanfiction' could be 'The Bond Beyond Rivalry'. In this fic, it explores the unexpected moments of understanding that develop between Inuyasha and Sesshomaru when they are in their younger forms. It delves into how their different personalities interact in a more innocent way compared to their adult selves.
One main theme is the exploration of family and rivalry. Since they are brothers, there's always that underlying family connection despite their rivalry. Fanfictions often play with this dynamic, showing how their relationship might have been different in their youth.
Maybe there are powerful dark sorcerers. These sorcerers could be trying to capture Naruto for their own evil purposes, like using his power to open a portal to a dark dimension or something, and Kurama has to protect him from them.
Maybe some of Tony's business rivals who have access to advanced technology. They could be jealous of Tony's relationship with Ultron and try to harm him. They might hire mercenaries or develop their own weapons to go after Tony. In this fanfiction, these rivals would be the main antagonists as they constantly try to break through Ultron's protection of Tony.
In many cases, the main antagonists can be other Slytherins who still hold grudges against Harry or are loyal to the pure - blood supremacist ideology. They might see Harry as a threat to their way of life. Also, some Death Eaters can be antagonists. They target Harry because of his role in Voldemort's downfall and would stop at nothing to get to him.
Perhaps there are some shinobi within the village who are jealous of Naruto's potential. They might try to harm Naruto in secret, and Hiruzen has to be vigilant against them. These shinobi could be from clans that have rivalries with the Uzumaki or those who just don't want a new powerful force emerging in the village.
It could be the Fire Nation remnants who still hold grudges against the airbenders. They might be trying to finish what they started years ago.
Some fanfictions might introduce new original antagonists. These could be powerful rogue ninjas who are after Sakura for her medical skills or some special power she might have in that particular fanfiction. They would then be the main threats that Itachi has to deal with to protect her.
There could also be humans who are afraid of the baby's mixed heritage. They might view it as an abomination and try to get rid of it. These humans could be part of a more superstitious group that believes demons and humans should never mix.