Well, one interesting aspect could be the exploration of their complex relationship in a more creative and unorthodox way. It might show different sides of their personalities that we don't get to see in the original works.
First, understand the characters well. Know their personalities, motives, and backstories from the original works. This will help you make their actions in the fanfiction believable.
Another theme is the clash of ideals. Gilgamesh has his own view of kingship which is often at odds with Saber's. This conflict of ideals can lead to some really intense and interesting storylines. For example, in some fanfictions, they have long debates about what it truly means to be a king.
One of the well - liked 'gilgamesh x saber fanfiction' is 'The Golden King and the King of Knights'. It beautifully weaves their personalities together in an alternate universe scenario.
The rich lore of the Fate/stay night universe. Fans are already invested in the world and the backstories of Saber and Gilgamesh. Fanfiction allows them to explore different scenarios that may not have been shown in the original works, like what if they had to team up against a greater enemy? Or what if they fell in love in a different timeline? All these ideas are appealing to fans and contribute to the popularity.
One often - overlooked source is Wattpad. While it's known for a wide variety of stories, there are some hidden gems of Saber x Gilgamesh fanfiction. However, it might take a bit more searching compared to AO3. You can also try joining Fate/stay night fan groups on Facebook or other social media platforms. Sometimes members share their own fanfictions or know where to find good ones in this pairing.
A plot twist could be a change in power dynamics. Suppose Gilgamesh loses his powers temporarily. Saber and Lancer would then have to protect him while also dealing with the enemies that are now more emboldened because of Gilgamesh's weakened state. This would force them to work together in new and unexpected ways, and also lead to character development as they face challenges they hadn't anticipated.
It could be about an epic battle. Gilgamesh is a very powerful and prideful character. Saber is known for her noble ideals and strong combat skills. Lancer is also a formidable fighter. Maybe the fanfiction is set in a situation where they are forced to fight each other in a multi - sided battle, with each trying to prove their worth and dominance.
A really interesting one is 'Love Across the Ages'. It delves deep into the pasts of both Gilgamesh and Saber. Their different cultures and experiences from their respective timelines create a complex and beautiful relationship as they learn to love each other despite their differences.
Since 'lemon' often implies sexual content, it's not really a positive or proper aspect to focus on in fanfiction. However, if we were to consider just the Gilgamesh part, it might involve taking the character's proud and powerful traits from the original story and perhaps distorting them in inappropriate ways in such fanfictions.
The cultural clash is an interesting aspect too. Naruto comes from a world of ninjas with its own values and traditions, while DxD has its own set of rules regarding devils, angels and fallen angels. Saber also brings in her own set of values from the Fate series. When these different elements collide in a fanfiction, it can result in really unique and thought - provoking situations.