You can start by looking on fanfiction websites like Archive of Our Own (AO3). There are often a plethora of fanfics related to different characters there. Just use the search function and type in 'Doctor Strange x Wanda Maximoff' and it should bring up relevant stories.
Some fanfics also focus on their individual character development in relation to each other. For instance, Wanda might learn more about controlling her powers from Doctor Strange, and he might learn about the emotional side of magic from her. This mutual growth is a popular theme in many of these fanfics.
Another theme is teamwork. Given their abilities, they can be a formidable team. Fanfics often show them coming together to face threats that neither could handle alone. For example, there are many stories where they fight against interdimensional villains. This theme also allows for character development as they learn to trust and rely on each other.
One great Wanda and Dr Strange fanfic is 'Mystic Encounters'. It explores an alternate universe where their powers interact in unexpected ways. The story has a great plot that keeps you hooked from start to finish.