His first potential ally could be the Lich. Since the Lich is all about destruction and chaos, just like Evil Finn in his evil state, they might team up. The Lich could provide some dark magic knowledge to Evil Finn to make him even more powerful.
Maybe Marceline the Vampire Queen could be an ally, but in a more complex way. She has her own dark side, and if Evil Finn managed to convince her that his plans would benefit her in some way, like getting more territory or power, she might join him for a while. However, her loyalty would always be a bit shaky as she also has good relationships with Finn and Jake.
The Fire Kingdom's Flame Princess could be an ally too. Her mastery over fire could complement Finn's powers in combat situations. Also, the various magical creatures in Ooo, like the gnomes or the fairies, could band together and support Finn. They might have their own ancient knowledge or magic that could be useful to Finn in his adventures with his godlike powers. Finn's kindness towards them in the past might make them more inclined to help him out.
In Adventure Time fanfiction, the Ice Princess could find allies in many characters. Lady Rainicorn could be one. Her flying abilities and combat skills could be very useful for the Ice Princess. BMO could also be an unexpected ally. BMO might have some knowledge or technology that could assist the Ice Princess in her adventure. Additionally, Flame Princess could be an ally under certain circumstances. If there is a threat that endangers both the Ice Kingdom and the Fire Kingdom, they could put aside their differences and work together.
Finn and Jake are obvious allies. They are always up for an adventure and helping those in need. Their bravery and unique skills would be very useful.
Princess Bubblegum could be one. She's smart, powerful, and has a long - standing relationship with Finn. Another could be Flame Princess. Their relationship has always been complex and full of chemistry. And maybe Marceline could also be part of it. She and Finn have a great friendship that could potentially develop into more in the context of a harem fanfic.
One possible plot could be Evil Finn trying to take over the Candy Kingdom. He might use his dark powers to turn the sweet inhabitants into his minions. Another plot could involve Evil Finn forming an alliance with other villains from the Adventure Time universe, like the Ice King in his more menacing moments. He could then plan to rule over all of Ooo together.
Maybe Kurama, the nine - tailed fox inside Naruto, could be an unexpected ally in a new way. Since they have a bond, Kurama could provide Naruto with extra power and insights when dealing with time - related challenges. For example, Kurama might have some ancient knowledge about the flow of time that could help Naruto in his time - patrolling duties.
Leon Kennedy could also be an ally. He has his own set of skills and experiences in dealing with dangerous situations. In a time - travel scenario, Leon might be able to provide different perspectives and resources. For example, if Chris travels to a time where Leon has already been, Leon could give him valuable information about the local threats and how to deal with them.
One potential new ally could be a powerful pirate from the past. For instance, a pirate who was known for his great navigation skills. Luffy could use his help to explore uncharted seas more easily.
One possible aspect of Finn losing his arm in fanfiction could be how it changes his physical capabilities. He might have to adapt his fighting style, perhaps relying more on his legs or using his remaining arm in more creative ways. For example, he could learn to use his sword one - handed in a whole new way that surprises his enemies.
Jon could find an ally in Arthur Dayne. Dayne was a great knight, and if Jon could prove his worth and show that he has knowledge of threats to come, Dayne might side with him. Also, Bran the Builder is a possibility. Given that Bran is such an important figure in Westeros' history and has some connection to the supernatural, he might be intrigued by Jon Snow's situation and decide to help him in his time - traveling endeavors.