Well, clearly Cammie and Joanna are the main characters. They are likely to be friends or sisters in the fanfic, and the story probably revolves around their relationship and the things they do together.
A common theme could be friendship. Since they are the focus, their bond might be explored deeply. Another could be growth, like how they grow as individuals while being friends.
Since I'm not familiar with the specific 'Shannon and Cammie' fanfiction, common themes could be friendship if they are friends in the original source. Maybe adventure if they are in a story that involves going on quests or exploring.
One great fanfiction is 'The Adventures of Cammie and Zach' which takes them on a thrilling spy mission outside of Gallagher Academy. It has really interesting character interactions.
Maybe in the fanfiction, Zach and Cammie are in a secret relationship. When Cammie discovers she's pregnant, they struggle with whether to keep it a secret or come out. There could be a lot of drama as they try to figure out how to balance their relationship, the pregnancy, and their own personal goals. For example, Cammie might be worried about how it will affect her future career plans, while Zach is trying to be the supportive partner she needs.
One interesting plot could be Cammie and Joanna going on an adventure together in a magical forest. They might encounter strange creatures and have to use their wits to get out of tricky situations.
One great 'Cammie Morgan and Zach Goode fanfic' is 'The Gallagher Adventure'. It continues their story with lots of action and mystery. They face new enemies together and their relationship develops further in this fic.
Some Tumblr blogs are dedicated to specific fandoms and might have zach and cammie fanfiction. You can search for relevant tags like '#zach and cammie' or '#zach and cammie fanfiction' on Tumblr. But be aware that not all Tumblr posts are in the form of a traditional story.