One common theme is the exploration of their relationship, whether it's adversarial or evolving into something more friendly. Another theme is Shego's own self - discovery, as she often has a complex backstory in fanfiction. Also, the power dynamic between them, with Kim being the hero and Shego the villain, is frequently explored.
The setting can also be important. Placing them in a different world or time period, like a steampunk version of their world or in an ancient civilization, can add a whole new layer of interest to the story. This allows for creative interpretations of their characters and how they would interact in such settings.
In a Shego - kidnaps - Kim - Possible fanfiction, the plot could be filled with high - stakes action. Shego might have some devious plan behind the kidnapping, perhaps related to a powerful artifact Kim was about to retrieve. Kim, being the resourceful hero, would surely find ways to outwit Shego even in captivity. This could lead to an exciting cat - and - mouse game within the story.
Another possibility is that Shego is being controlled or influenced by some external force. This external entity might have a vendetta against Kim and is using Shego as a tool to carry out the killing. Shego, being in a subservient position to this force, has no choice but to try and kill Kim.
Their relationship would become much more complex. They would have to put aside their differences and work together for the sake of the baby. Instead of being just enemies, they would become reluctant partners in parenting.
One great 'Kim Possible x Shego' fanfiction is 'The Unlikely Alliance'. It explores a scenario where Kim and Shego have to team up against a new common enemy. The story does a great job of delving into their complex relationship, showing how their initial animosity gradually gives way to a grudging respect.
The contrast between their characters. Kim is the good - hearted hero while Shego is often the villain. When they kiss, it's like two opposite worlds colliding. It makes readers wonder how their relationship will develop further, will Shego turn good? Or will Kim be influenced by Shego's more devious side?
A common theme is the idea of redemption. Shego, being a villain, often has a journey towards redemption which is intertwined with her relationship with Kim. For example, Kim might be the catalyst for Shego to change her ways.
There are quite a few. Some might focus on their initial enmity gradually turning into love. For example, a story could start with their battles and then show how they start to understand each other on a deeper level. Others might be set in alternate universes where they are already in love from the start and face different challenges together.
Shego might kidnap Kim Possible in fanfiction because of jealousy. Maybe Kim always gets the credit and Shego wants to show that she's just as capable. So, she kidnaps Kim to prove a point.