Well, one possible plot could be that an Assassin from the Assassin's Creed universe somehow gets transported to the world of Strike Witches. He could find himself in the middle of the battle against the Neuroi. Using his assassin skills like stealth and parkour, he could start to help the Witches in unexpected ways. Maybe he teaches them some new combat techniques based on his own training, and in return, the Witches introduce him to their unique magic - powered flight abilities.
Perhaps there is an ancient artifact in the Strike Witches world that has a connection to the First Civilization from Assassin's Creed. The witches don't understand its true power, but an Assassin, who has some knowledge of these artifacts, arrives. They then have to work together to figure out how to use the artifact either to defeat the Neuroi once and for all or to prevent it from falling into the wrong hands. The Assassin could use his Eagle Vision to uncover secrets about the artifact and the witches' world.
The characters would likely start off being cautious of each other. The Strike Witches, used to their own style of fighting and magic, might be a bit skeptical of the Assassins' more grounded combat skills. But as they get to know each other, they could start to exchange knowledge. For example, Ezio from Assassin's Creed could show the witches how to better use their daggers in close - combat situations. In return, the witches could take Ezio on flights, showing him their world from above. They would gradually build trust and respect as they face common enemies together, like the Templars or the Neuroi depending on the situation.
One possible plot could be that Sherlock is investigating a series of murders that seem to be related to an ancient Assassin's Creed artifact. The assassins are also on the hunt for this artifact, thinking it holds great power or secrets. Sherlock's deductive skills and the assassins' combat and stealth abilities would clash and cooperate as they both try to figure out who the real killer is and what the artifact does.
One possible plot could be that Ezio, in his quest for knowledge or to face a new threat, somehow discovers a way to travel through time and space to meet Altair. Maybe he needs Altair's wisdom on a particular mystery related to the Assassins' Creed that only the founder might know.
One possible plot could be that the ninjas from Naruto world discover the existence of the Strike Witches. They could initially be in conflict due to differences in their fighting styles and goals. But then, they realize they have a common enemy, like an ancient dark force that threatens both their worlds. They then combine their powers, with the Naruto characters using their jutsus and the Strike Witches using their magic and flight abilities to fight this new threat.
One possible plot could be that a pony discovers an ancient artifact in Equestria that has Assassin's Creed - like powers. Maybe it gives the pony the ability to blend into the shadows like an assassin and jump incredibly high. The pony then has to use these new powers to protect Equestria from an unknown threat that is slowly emerging and wants to take over the magical land.
One interesting storyline could be about a new Witch joining the squadron and having to prove herself. She might struggle with fitting in at first, but through her unique skills and determination, she becomes an integral part of the team.
Well, it could be really cool. Maybe the assassins would have some really wacky adventures in a cartoon - like world. Their stealth skills could be used in some crazy, colorful settings, like sneaking through a candy - land type of place in the cartoon world while being chased by cartoonish Templars.
You could write a fanfic about how the Syndicate interacts with other Assassin's Creed groups around the world. The Frye twins might receive a message from an Assassin cell in another country asking for help. This could involve traveling to a new location, facing new enemies, and learning about different Assassin traditions. It would expand the world of Assassin's Creed Syndicate and show how the fight against the Templars is a global one.
One idea could be that Naruto, with his ninja skills, gets transported to the Assassin's Creed world. He might mistake the Assassins for a new type of ninja. He could then start learning their ways, like using the hidden blade in combination with his own jutsu. Another plot could involve Naruto teaming up with an Assassin to take down a powerful enemy that is causing chaos in both the Naruto universe and the Assassin's Creed universe.
A really interesting plot would be if the Akatsuki from Naruto were somehow related to the Templars in Assassin's Creed. Maybe they share a common goal, or there's a power struggle between them. Or, imagine a scenario where Sasuke becomes an assassin. He already has the skills and the brooding personality. He could use his Sharingan to enhance his assassin abilities, like predicting enemy moves better.