Well, perhaps a plotline where an Eldar character forms an unlikely alliance with a human. They have to overcome their initial distrust and cultural differences to fight against the Covenant. This could involve sharing their unique technologies and combat strategies. For example, the Eldar might teach the humans how to better use psychic defenses against the Covenant's psychic attacks.
A possible plotline is that the Eldar discover some ancient weapon or technology during the war that has the potential to tip the balance of the conflict. However, they are conflicted about whether to use it or keep it hidden. On one hand, using it could end the war in their favor, but on the other hand, it might have unforeseen consequences for the galaxy as a whole. So they have to weigh the risks and rewards while also dealing with the humans and the Covenant closing in on their discovery.
Another possible main character could be a Covenant Elite. They are often leaders in the Covenant military structure. Having a Covenant Elite in the story would add an interesting dynamic as they are the enemy of the humans, but might also have some interactions or conflicts with the Eldar, perhaps over resources or territory during the war.
In some fanfictions, there might be a story where a human discovers an ancient Eldar artifact. This leads to an Eldar coming to claim it. Instead of fighting, they negotiate and end up on a journey together to understand the true power of the artifact. Along the way, they learn about each other's cultures and form an unlikely bond.
One common theme is cultural exchange. Since humans and eldar have very different cultures, it's often explored how they learn from each other in these fanfics. For example, in many stories, humans might be introduced to eldar art forms or spiritual beliefs, and vice versa.
One interesting plotline could be about the different factions in Equestria warring over scarce resources. For example, the ponies of Canterlot might be fighting with the ponies from Ponyville due to a shortage of magic crystals. Another could be a story where an unlikely hero emerges from a small group of ponies who were previously overlooked in the war effort. Maybe a group of earth ponies discovers a new way to defend their land against the more magically - inclined opponents.
One interesting plotline could be a Dark Eldar raiding party stumbling upon a lost Imperial relic. They see it as a valuable item to either use for their own dark purposes or sell on the black market in Commorragh. Another could be a story about a Dark Eldar wych who tries to rise through the ranks by challenging powerful rivals within her own cult.
One interesting plotline could be Sasuke and Sakura exploring their new relationship in a post - war world filled with rebuilding. They might face challenges in their relationship due to the traumas of war. Another could be them working together to heal the wounds of the village, both physically and emotionally. Maybe they discover new aspects of each other as they do so.
Well, perhaps in this fanfic, Emiya has to face some moral dilemmas during the fourth war. He might be forced to make choices between saving civilians and achieving his goals in the war. Also, his relationship with his Master could be explored in a new light. What if his Master has a hidden agenda that conflicts with Emiya's own sense of justice? That would surely add a lot of drama to the story.
Another plotline might involve a young pony who has the ability to communicate with the crystals. This pony could then be caught in the middle of the war, with both sides trying to use their unique talent for their own purposes. They might struggle with loyalty and the right path to take.
One interesting plotline could be the resurrection of long - forgotten shinobi who play unexpected roles in the war. For example, some shinobi from the early days of the Hidden Villages might return with unique jutsu that change the tide of battle.
One interesting concept could be the unique character relationships. In RWBY Covenant fanfic, characters might form new alliances or rivalries that deviate from the original series. For example, characters from different teams could team up in unexpected ways to face a common threat.