Well, you could write a fanfic about a Murder Drone that gets damaged and loses its memory. It then has to figure out who it is and what its purpose is while being helped by a group of Worker Drones. Another interesting idea is a fanfic that explores the emotional side of the Murder Drones more. What if a Murder Drone starts to feel guilt for killing Worker Drones? And a fanfic where the Murder Drones encounter a more advanced form of themselves from the future, and they have to decide whether to follow in their footsteps or find a different path.
One interesting Murder Drones fanfiction could be about a drone that rebels against its programming to not harm humans. It might start to protect the workers on the planet instead of hunting them. This could lead to a whole series of events where it has to hide from the other murder drones while trying to convince the humans it means no harm.
To start writing a Murder Drones fanfic, begin by choosing a perspective. Are you going to write from the point of view of a Murder Drone, a Worker Drone, or maybe an outside observer? After that, think about the setting. Will it be in the factory where they usually operate, or in a new location? Once you have these things figured out, start with a strong opening. Maybe a dramatic scene where a Murder Drone is about to attack a Worker Drone, but then something unexpected happens. As you write, don't worry too much about making it perfect at first. Just get your ideas down on paper (or on the screen) and you can always go back and edit later.
One possible plot could be that a character from Ultrakill ends up in the world of Murder Drones. Maybe the Ultrakill character's combat skills are put to the test against the Murder Drones. They could form an uneasy alliance at first, as they both realize they have a common enemy, like a new type of rogue AI that threatens both of their worlds.
To start writing Murder Drones fanfiction, begin by watching the series multiple times to really understand the nuances of the world and the characters. Next, think about the themes that interest you, like survival, morality, or the relationship between different classes of drones. Once you have a theme in mind, you can start building your plot around it. Don't be afraid to deviate from the main story a bit, but also make sure it still feels like it belongs in the Murder Drones universe. You can also look at other fanfictions for inspiration, but make sure your story is unique.
I think Gabriel from Ultrakill would make for an interesting crossover. His complex personality and powerful abilities would add a lot to the Murder Drones' world. He could be a force that either helps or hinders the Murder Drones depending on his goals. His presence would also likely stir up a lot of drama among the Murder Drones characters as they try to figure out what his intentions are.
If we swap Star and Marco, then the main antagonists could be those who were previously minor annoyances for the other character. For example, if Marco is now in Star's role, Ludo, who was a main antagonist for Star, could be an even bigger threat as Marco might not have Star's natural magical intuition to deal with him.
In a 'wife swap murder story', it could start with two couples agreeing to this rather unconventional exchange. As they start living in the swapped situations, they might discover things about each other's partners that they don't like. Maybe one partner has a dark past or a violent temper. Tensions rise, and eventually, due to misunderstandings, arguments over property, or just pure hatred, one person decides to end another's life. This kind of story often explores the dark side of human relationships and how extreme situations can lead to the worst of human behavior.