Well, there are many possibilities. Maybe he'll become a legendary figure and bring lasting change to his world. Or perhaps there'll be a bittersweet ending with sacrifices and new challenges.
Another theme is their growth together. Fanfictions often show how they help each other to become better versions of themselves. Like Adolin learning to be more in touch with his emotions because of Shallan, and Shallan becoming more confident with Adolin's support. This is often shown through different adventures or challenges they face as a couple.
One common theme is their relationship development. Writers often explore how they grow closer and understand each other better. Another is adventure. They might be put in new and exciting situations together.
One great Adolin and Shallan fanfiction is 'A New Beginning' which delves into their relationship after the events of the main series. It beautifully portrays their growth as individuals and as a couple.
You can start by looking on Archive of Our Own (AO3). It has a large collection of fanfictions about various topics, including Adolin and Shallan. Fanfiction.net is also a good place. There are many talented writers who post their works there.
Well, the Teenage Mutant Ninja Turtles cartoon is a classic. Rat Man, though? Usually not related. Maybe you saw some fan art or a creative interpretation that made you think there was a connection, but in the mainstream, they don't typically intersect.
Common themes could include the hero's journey of finding their true purpose again after a fall, and also exploring the complexity of good and evil. Another theme might be the impact of society's judgment on the fallen hero.