Well, you could write a fanfiction where the AllSpark plays a more central role in Darkness Rising. Maybe it has some sort of hidden message or power that the Autobots and Decepticons are unaware of. Another idea is to focus on the relationship between Optimus Prime and Megatron during this time. What if there was a moment where they could have reached an understanding but something got in the way? And for a more character - driven story, you could explore how the events of Darkness Rising affected Bumblebee on an emotional level, and how he grows from it.
One idea could be exploring the backstories of the Decepticons in more detail. For example, how did Starscream become so power - hungry? Another could be creating an alternate timeline where the Autobots lose in Darkness Rising and have to find a new way to fight back. Maybe a story about a human getting caught in the middle of the Autobot - Decepticon war and having to use their wits to survive.
A cool plot for Transformers Prime: Darkness Rising fanfiction could be a story where the Autobots discover a hidden power within themselves that was unlocked during the events of Darkness Rising. This power could change the course of the war. Or perhaps a fanfiction about a Decepticon who has a change of heart and tries to help the Autobots, but has to face the wrath of his former comrades. You could also write about a new character, a young Transformer, who emerges during the chaos of Darkness Rising and has to choose a side.
Another plot could involve a group of characters from different factions in 'the darkness rising' fanfiction having to team up. They might not trust each other at first, but as they face greater threats from the darkness, they learn to work together. For example, a former enemy might have crucial knowledge about a weakness in the dark forces.
One idea could be a story where Optimus Prime has to team up with an unlikely ally from another planet. Maybe a small, scrappy alien that has some unique abilities that can help in the fight against the Decepticons. This would add an interesting dynamic as Optimus has to learn to work with a very different being.
A plot idea could be that Optimus Prime time - travels. He goes back to the beginning of the war between the Autobots and the Decepticons and tries to prevent it from starting. Along the way, he meets younger versions of his friends and enemies and has to deal with the consequences of changing the past. Or perhaps, Optimus is trapped in a dimension where the laws of physics are different. He has to adapt quickly and find a way back to his own world while also facing off against new threats.
One idea could be exploring the backstory of a minor character in the Darkness Rising Series. Maybe a character who only had a brief appearance but had a lot of potential. For example, you could create a story about how they got involved in the events of the series in the first place.
Another plot idea could be a 'what if' scenario. What if the main character made a different choice at a crucial moment? This could lead to a completely different outcome for the story, like different alliances forming or new threats emerging.
One idea could be that Optimus Prime travels back in time to prevent the creation of a powerful Decepticon weapon. This would change the whole course of the war. Another could be a human character traveling to the future of the Transformers' world to see how the conflict ends and bring back knowledge to change the present situation. And perhaps a Decepticon time - travels to the past to rewrite history and make themselves the rulers from the very beginning.
A great plot could be that a powerful artifact from the Transformers universe is found on Earth and it has the power to either save or destroy the planet. The Avengers initially think it's a new weapon or power source, while the Autobots know its true origin. They have to convince the Avengers of its importance and together protect it from the Decepticons and any other villains who want to use it for their own evil purposes.
One idea could be a crossover where the characters from Transformers Prime end up in the Beast Wars universe. They could form alliances with the Beast Wars characters to fight a common enemy like the Predacons. Another idea is to explore the origin stories of some of the Beast Wars characters within the context of the Transformers Prime lore. Maybe there's a hidden connection between a Prime character and a Beast Wars character that could be uncovered.
Perhaps a fanfiction where Soundwave gets damaged in a fight and Megatron has to make a difficult decision about whether to risk everything to save him. It could explore their loyalty to each other. Or a plot where they are betrayed by one of their own and they have to work together to expose the traitor.
One idea could be an OC Decepticon who has a unique ability like being able to control electricity. The plot could revolve around this OC trying to prove their worth to Megatron by taking on difficult Autobot missions. For example, they target an Autobot outpost that holds a powerful energy source. The OC's journey to the outpost, facing Autobot defenses and internal Decepticon rivalries along the way, would make for an exciting story.