Well, Mirajane could be part of the harem. She has a motherly side and would want to comfort Natsu if he was betrayed. Levy is also smart and resourceful, and she could use her skills to help Natsu out. Juvia, with her obsessive love for Gray, might transfer some of that intense loyalty to Natsu if she saw him in a desperate situation after being betrayed.
One idea could be that Natsu is betrayed by his closest friends in the guild, and in his time of despair, a group of female characters form a harem around him to help him heal and regain his confidence. For example, Lucy could be the first to reach out, using her celestial magic to show him there are still things to fight for. Another plot could involve Natsu being betrayed by a powerful dark mage who manipulates his guild against him. The harem, which might include Erza, Wendy, and others, then trains Natsu in secret to become stronger and take revenge on those who wronged him.
Lucy could be a likely member. She has a close relationship with Natsu already in the original story. Another one could be Erza. Her strong and independent nature might be attracted to Natsu's new godlike power. And Wendy, being young and impressionable, might also be part of the harem as she looks up to Natsu.
Well, one possible plot could be that Natsu is betrayed in his own world and somehow ends up in the DxD Highschool universe. Maybe he has to deal with the different power dynamics and politics there while also trying to figure out who betrayed him in his original world. He might form new alliances with the characters in DxD Highschool and use his unique abilities to survive and seek revenge for his betrayal.
One great 'Fairy Tail Natsu x Harem' fanfiction is 'A New Adventure'. It has an interesting plot where Natsu finds himself in various unexpected situations with the harem members. The character development is well - done, especially how Natsu's relationships with each of the harem characters evolve over time.
One popular 'natsu x juvia harem fanfiction' is 'The Unlikely Bonds'. It has an interesting plot where Juvia finds herself in a harem - like situation with Natsu. The author does a great job of exploring the characters' emotions and the unique dynamics that come with such a pairing.
Natsu, being a fiery and passionate character, would likely have a rather intense interaction with the DxD Highschool characters. He might be initially distrustful due to his recent betrayal. But some of the more friendly characters in DxD, like Issei's group, could try to befriend him. Natsu would probably show off his powers a bit, which would either impress or intimidate the others. And there could be some clashes in personalities, for example, Natsu's straightforwardness against some of the more devious DxD characters.
It could be Gray. Although they are friends most of the time, in a fanfiction, there could be a plot twist where Gray's envy of Natsu's abilities gets the better of him, and he betrays Natsu. Maybe Gray thinks that by getting rid of Natsu, he can be the top mage in their guild.
She might become angry at first. Natsu is known for her fiery temper. So, she could lash out at the person who betrayed her, confronting them directly and demanding an explanation.
A more detailed title could be 'In the World of Fanfiction: Fem Natsu's Journey After Betrayal'. This title not only mentions the main idea of fem natsu being betrayed but also implies that there will be a journey or development in the fanfiction that follows the betrayal.