Yes, he could be in certain fanfictions. Fanfiction allows for a lot of creative freedom. Maybe the writer wants to show Jack's journey from being a Unicron fan to realizing the true nature of this evil entity. It can be a great way to explore character development. Or perhaps it's part of a more complex plot where Unicron has some sort of allure that Jack initially falls for, but then has to make a choice between his newfound interest and his loyalty to his Autobot friends.
One interesting female Decepticon fanfic could be about a female Decepticon's journey to find her true place within the Decepticon ranks. It might explore her relationships with other Decepticons, both positive and negative.
One reason could be the allure of power. Unicron is an extremely powerful entity, and Jack might be attracted to that power. Maybe he feels that the Autobots' power is not enough to deal with the threats they face, and he sees Unicron as a potential solution. Another reason could be a sense of rebellion. Jack might be tired of the Autobots' rules and way of doing things, and Unicron represents a break from that.
One idea could be a crossover with a modern military setting. The Decepticons could be trying to steal advanced military technology. Another is a crossover with a post - apocalyptic world where they are competing with other factions for resources. A third could be a crossover with a magical realm, with the Decepticons trying to harness the magic for their own evil purposes.
One common theme is redemption. Many fanfictions explore the idea of a Decepticon OC trying to break free from the evil ways of the Decepticon faction and find a new path. For example, an OC might realize the destruction they've caused and seek to make amends.
One possible storyline could be that after years of war, an Autobot and a Decepticon realize the futility of their fight. They secretly meet and start to communicate. They find that they both have similar desires for their home planet, Cybertron. So they decide to work together to bring about peace, facing opposition from both their factions.
A common theme is the clash of two different worlds. The magical world of Harry Potter and the technological, war - based world of the Decepticons are so different that when they meet, there's bound to be conflict. Another theme is the idea of power. Both universes have their own forms of power, magic in Harry Potter and advanced weaponry in the Decepticons. Writers often explore how these powers interact and which is more dominant.
In a Transformers Prime fanfiction, a human could befriend a Decepticon who is disillusioned with the evil ways of their faction. Together, they try to find a way to bring peace between the Autobots and Decepticons. This would involve many challenges as both sides are distrustful of the other. The human's unique perspective might be the key to making it all work.
There could be fanfics where a female Decepticon is in a love - hate relationship with an Autobot. It's a complex situation as they are enemies, but the attraction is there. This kind of fanfic can explore themes of loyalty, betrayal, and the power of love across the divide. You can often find such concepts on various fanfic websites by using relevant search terms.