A possible plot is that there's a mysterious force or curse in the ninja world that causes Hinata to become pregnant. Gaara, being a powerful and empathetic leader, decides to protect her. The story could revolve around their search for a way to break the curse if it's harmful, or how they prepare for the new life if it's a normal pregnancy. It could also involve their friends from their respective villages helping them out and the cultural exchange that comes with it, since the Sand and Leaf villages have different traditions.
One idea could be a story where Gaara and Hinata are brought together by their respective villages for a diplomatic mission. During this time, they develop a deep connection and Hinata discovers she's pregnant. The plot could then focus on how they navigate the political implications within their villages, as well as their personal feelings. Another plot could be a more AU (alternate universe) setting where they are ordinary people, not ninja. They meet in a small town, fall in love, and Hinata gets pregnant. They then have to deal with the challenges of starting a family in a non - ninja world, like finding stable jobs and a place to live.
Well, a plot could start with Hinata being sent on a mission near the Sand Village. There, she and Gaara cross paths and start to collaborate on some local issues. As they spend more time together, feelings develop and Hinata soon realizes she's pregnant. The fanfiction could then explore how Gaara, who has had a difficult past, learns to embrace fatherhood. And how Hinata, with her gentle nature, helps him through his insecurities. The story could also include how their relationship affects the relationship between the two villages, whether it brings more peace or some jealousy and rivalry among other ninja.