Another possibility is that a new and mysterious group of shinobi emerges. They are after Rin because she has a special chakra that they need for some dark ritual. Obito has to figure out their motives and stop them before they can harm Rin. This new group could have some really strange and dangerous powers that Obito has never encountered before.
It could be rogue ninjas who have a vendetta against Obito or Rin. They might be seeking revenge for something Obito did in the past or they might want to use Rin as bait to lure Obito into a trap. These rogues could be very powerful and have unique abilities that make it difficult for Obito to save Rin.
One potential antagonist could be the Hidden Mist ninjas. They are known for their ruthless tactics, and they might target Rin for some strategic reason. Maybe they think she has valuable information about Konoha's defenses.
One possible plot could be that Obito arrives just in time to save Rin from being captured by enemy ninjas. He uses his Sharingan abilities to outwit the enemies and then escapes with Rin. Maybe he has to fight through a horde of strong opponents, but his determination to save Rin gives him the strength to persevere.
One potential antagonist could be Orochimaru. He is always on the lookout for powerful individuals to experiment on or use for his own gain. Mikoto, being an Uchiha, has valuable kekkei genkai that Orochimaru might be interested in. Another could be a group of anti - Uchiha extremists who see Mikoto as a symbol of the Uchiha power they want to destroy.
One idea could be that Kakashi travels back to the moment right before Obito's 'death'. He uses his knowledge of future events to prevent the cave from collapsing on Obito. Then, together they are able to rescue Rin more effectively and change the whole tragic course of events.
Their relationships would become much stronger. Kakashi would no longer have to bear the guilt of Obito's 'death' and Rin's loss. Obito would be extremely grateful to Kakashi for saving his life and would likely view him as an even greater friend and ally. Rin would also be happy to have both of them by her side, and they could form a very tight - knit team.
One great Rin and Obito fanfiction is 'A Different Path'. It explores an alternate universe where Obito returns to the village earlier. The story delves deep into their relationship, showing how they might have interacted and grown together. The character development is really well - done, and it gives a fresh perspective on their bond.
It could be Team Rocket. They are always causing trouble for Ash, and it would make sense for them to be the ones behind Ash's predicament in this fanfiction. They might have a new evil plan to capture Ash and Sceptile comes to the rescue.
One potential antagonist could be a rogue ninja who also has aerokinesis but uses it for evil. He might be jealous of Naruto's natural talent and try to undermine him at every turn. Another could be a powerful sorcerer from a far - off land who has come to the Naruto universe specifically to find someone with aerokinesis to either steal their power or use them as a tool. There could also be a group of anti - ninja who see Naruto's new power as a threat to their plans of overthrowing the ninja system and they try to eliminate him.
A good Rin and Obito fanfiction should have strong characterizations. Since we already know the basic personalities of Rin and Obito from the original story, the fanfiction should build on that and show new aspects of their characters. For example, how Obito might have changed if Rin had survived.
One possible antagonist could be a rogue ninja from another village. They might target Tenten for her unique weapon - handling skills, thinking they can use her to create powerful weapons for themselves. Naruto would then have to face this ninja to save Tenten.
It could be some rogue ninja clans that are after Karin for her special abilities. They might see her as a tool to gain power or information. For instance, a clan that specializes in blood - related jutsu might want to use Karin's unique blood - sensing ability for their own evil purposes.