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.
Common themes include dealing with the consequences of their powers. Both characters have had instances where their powers got out of control. Fanfiction often delves into how they manage this, whether it's Strange trying to control the time - related consequences of his magic or Wanda handling the chaos her magic can create. Another theme could be their place in the greater superhero community, and how they interact with other heroes through their relationship with each other.
Yes, there are many. You can find some on fanfiction websites like Archive of Our Own. Some stories might explore their relationship in different ways, like teaming up against a powerful enemy or having a deep friendship that develops over time.