One common theme is the power dynamic. Since Geto has strong powers in the Jujutsu Kaisen universe, exploring how that affects his relationship with the reader can be interesting. For example, he might protect the reader using his powers.
One common theme is friendship. Their bond, whether it's growing stronger or falling apart, is often explored. For example, many fanfictions show how they supported each other in their early days as sorcerers.
One great fanfiction is 'The Bond Beyond'. It delves deep into the complex relationship between Gojo Satoru and Geto Suguru. It shows their early days at Jujutsu Tech, their training together, and how their ideologies start to diverge. The author does an excellent job of portraying their individual personalities and the strong connection they once had.
Another option is to check out Tumblr. There are often fanfic recs and some users post their own Geto x Reader fanfics. Follow some relevant tags like #Geto x Reader or #Jujutsu Kaisen fanfic to find new and interesting stories.