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.
A group of independent adventurers who have their own grudges against Fairy Tail. They could join forces with Lucy to carry out their revenge plans.
She might use her celestial spirits in unexpected ways against the guild. For example, she could have her spirits create illusions to embarrass the members.
Well, if we're following the common ships in the Fairy Tail fandom, Natsu could be a likely candidate. Their relationship is very close and there are many hints of a deeper connection between them in the original series. It would make for an interesting story as Natsu would have to grow up a bit to take on the role of a father.
Yes, there are likely fanfictions with such a plot. Fanfiction often explores different scenarios and relationships within the 'Fairy Tail' universe. However, not all of them are well - written or in line with the original spirit of the series.
One potential kidnapper could be a dark wizard who wants to use Lucy's Celestial Spirit keys for their own dark magic. Another could be a rival guild member who is jealous of Lucy's power and wants to steal her keys to gain an advantage in the magical world. It could also be someone from Lucy's past, like an old acquaintance who has turned bad and has some grudge against her.
It could be an evil sea witch. She might be jealous of Lucy's newfound mermaid powers or her connection to the Fairy Tail guild. The sea witch could try to steal Lucy's powers or use her to gain access to the land world.
It could be a new character introduced specifically for the fanfiction. Maybe a powerful mage who has a vendetta against Lucy and somehow manages to drive her out of Fairy Tail by causing trouble for her within the guild.
Natsu would be a likely character to comfort Lucy. He's her close friend and often there for her. His warm and energetic nature could cheer her up. Another one could be Erza. Erza is strong and reliable, and she has a soft spot for her guildmates. She would probably offer Lucy some wise words and a shoulder to cry on. Happy, the little blue exceed, might also be there. His cute antics and friendly presence could make Lucy feel better.
Natsu would be a very likely character. Since he and Lucy are close friends in the original story, if Lucy travels back in time, Natsu might be one of the first people she seeks out. They could have adventures together in the past, with Natsu being his usual impulsive self and Lucy having to keep him in check with her knowledge of the future.
Gray could also be a possibility. He and Lucy have a friendly rivalry and there are moments where their relationship could be interpreted as having deeper feelings. It would be an interesting twist if Gray was the father, as it would change the dynamics between him, Natsu, and Lucy in the fanfic.
Other dragon slayers could be her potential allies. They might recognize her skills despite her being a princess. Also, magical creatures who are enemies of the dragons, like phoenixes or griffins, could side with her. And some wise old wizards or witches who see her potential might offer their help.