One common theme is Diego's struggle with his identity. In many fanfictions, he is trying to figure out who he is outside of being part of the Umbrella Academy. Another theme is his relationship with his family, especially his siblings. There are often conflicts and reconciliations explored.
In fanfiction, Vanya and Diego often have complex relationships. Sometimes they start as comrades who slowly realize they have deeper feelings. Their powers and personalities play a big role. Vanya's quiet strength and Diego's more brash nature can create interesting dynamics.
One popular trope is the 'enemies - to - lovers' scenario. Diego and Vanya might start off at odds due to their different personalities or past misunderstandings, but then gradually fall in love. Another is the 'protector' trope, where Diego feels the need to protect Vanya from various threats.
One great piece of Diego fanfiction is 'The Adventures of Diego in the Umbrella Academy'. It delves deep into Diego's character, exploring his complex relationship with his siblings while also taking on new missions within the academy's universe. Another interesting one is 'Diego's Hidden Powers', which focuses on Diego discovering new and unexpected abilities.
Their powers are very different, Diego with his ability to manipulate knives and Klaus with his ability to communicate with the dead. This contrast creates an interesting dynamic in fanfiction as they can complement each other's skills. For example, Klaus might get information from the dead that helps Diego in a fight.
There could also be a fanfiction where Diego and Klaus get transported to an alternate universe. In this new world, they have to find a way back while also dealing with versions of their siblings that are very different. This would be an exciting adventure that tests their bond and forces them to adapt quickly.