The unique character of the OC. If the OC has distinct abilities, like being able to control a special kind of chakra or having a rare skill, it can make the story more engaging. For example, an OC who can communicate with animals in a world like Naruto's can bring new plotlines when paired with Gaara.
The development of the relationship between Gaara and the OC is key. It should be more than just instant love. A good 'gaara x oc fanfic' shows how they slowly understand each other, overcome differences, and grow together. Maybe the OC initially fears Gaara because of his reputation, but as they interact more, they see the real person inside.
The integration into the Naruto universe also matters. A great fanfic will use the existing lore, like the different ninja villages, jutsu, and politics. For instance, if the story is set during a time of tension between Suna and Konoha, how Gaara and the OC navigate this situation while developing their relationship can be really fascinating.