One common theme is sisterly love and rivalry. Since they are sisters in the Mortal Kombat lore, fanfics often explore the complex relationship between them. Another theme could be redemption, especially for Mileena who has a more villainous side in the original story.
You can start by looking on fanfiction websites like Archive of Our Own (AO3). It has a large collection of fanfics for various fandoms, including Mortal Kombat where Kitana and Mileena are from. Another option is FanFiction.net. Just search for 'Kitana x Mileena' in the search bar and you'll get a list of relevant stories.
For Mileena, you can focus on her internal struggle. She has this wild, Tarkatan side but also wants to be more like Kitana. So, show her trying to control her instincts. For Kitana, explore her leadership qualities. Maybe she takes Mileena under her wing and teaches her how to be a better fighter and a more noble person.
Well, it could be about their sisterly bond. Maybe they start off at odds due to the whole Tarkatan side of Mileena, but then they discover a common enemy and have to work together. They could learn more about their shared heritage and grow closer as sisters in the process.
It's a bit complicated. Sometimes certain elements of the MKX comics are incorporated into the overall canon, while others might be more standalone or supplementary. It really varies based on the larger narrative framework and how it all fits together.
It depends on how you measure it. If you count the main plotline, it's a decent length. But if you include all the side quests and character development, it can feel quite extensive.