Makarov could be her mentor. He is the master of the Fairy Tail guild and has a vast amount of magical knowledge and experience. He could guide Lucy in exploring deeper aspects of her celestial spirit magic, teaching her how to better communicate with her spirits on a more profound level, which would in turn make her stronger when summoning and using their powers.
Sasuke could be her mentor. He has extensive knowledge of various weapons and his skills with bladed weapons are well - known. He could teach Naruko how to hold the tanto properly, the different stances for attack and defense, and how to use chakra with it for enhanced strikes.
It could be an older, more experienced android. One that has been through a lot and has a deep understanding of both the android world and human society.
The main characters are clearly Natsu and Lucy. Natsu, with his fiery and impulsive nature, is likely to be there to encourage and sometimes push Lucy in her journey to get stronger. His dragon slayer magic might also play a role in training her or protecting her during dangerous situations.
Maybe Dumbledore in a more in - depth way. In the original series, Dumbledore was somewhat mysterious in his teachings. Here, he could really focus on Harry's growth and give him proper training, perhaps in the art of divination and understanding prophecies better.
Well, it could be that the mentor in 'Connor gets a mentor fanfiction' is someone with a lot of experience in a particular field that Connor is interested in. For example, if Connor wants to improve his combat skills, the mentor could be an expert fighter. The mentor could show Connor new techniques, and also help him develop strategies for different situations. This would not only make Connor stronger but also more confident in his abilities.
It could be another character who is jealous of Lucy's new powers. Maybe a classmate or a rival in her community who wants those powers for themselves and will do anything to take them from Lucy.
Another powerful mage from a rival guild. This mage could see Lucy's revenge as an opportunity to weaken Fairy Tail and offer their assistance.
One possible plot could be that Lucy trains intensively with Natsu by her side for support. They might go on special quests to find magical items or training methods that can enhance Lucy's power. For example, they could search for an ancient spell book that has the power to unlock new celestial spirit powers for Lucy.
Perhaps a mysterious figure who has been observing Sakura from the shadows. This person, for some reason, decides to train her to be an assassin. He could have some ulterior motives, like using Sakura for a grander plan, but still teaches her valuable skills along the way.
Maybe Kakashi could still be his mentor. Even though One for All is a new and different power, Kakashi is very experienced. He could help Naruto understand how to incorporate this new power into his overall combat strategy. Kakashi would be able to analyze the power and give Naruto advice on how to train with it while still relying on his ninja skills.