One interesting concept could be a crossover with a fantasy world like Middle - earth. Imagine an assassin having to navigate the perils of Mordor while still maintaining their creed. Another concept might be a crossover with a historical event not directly related to the main Assassin's Creed timeline, such as the sinking of the Titanic, where assassins could be on board for a secret mission.
Writing a good Assassins Creed crossover fanfic is all about balance. You have to balance the elements of Assassin's Creed, like the hidden blade and the assassin's parkour skills, with the elements of the crossover. Let's say it's a crossover with a detective story. You can't overshadow the detective's investigation skills with the assassin's combat abilities. Also, create believable relationships between the characters. If an assassin teams up with a detective, there should be some initial mistrust that gradually turns into respect and cooperation as they work towards a common goal.
One interesting scenario could be Ezio meeting Altair. Ezio, being from a later era, could learn a great deal from Altair's wisdom. He might be in awe of Altair's original Assassin skills and the way he built the Assassin Brotherhood in the Holy Land. Altair, on the other hand, could see the evolution of the Brotherhood through Ezio and how it has adapted to new challenges in Italy. Another scenario could be Connor meeting Edward Kenway. Connor, who is very much about protecting his land and people, could have an interesting conversation with the more pirate - like Edward. Edward could perhaps tell Connor about his adventures on the high seas and how he came to be an Assassin, which might give Connor a different perspective on the Brotherhood's reach and purpose.
When Ezio from Italy meets Altaïr from the Holy Land, their cultures could blend in interesting ways. Ezio's more flamboyant Italian style might initially seem at odds with Altaïr's more austere Arabian ways. But they could find common ground in their Assassin training. Ezio could introduce Altaïr to the finer arts of Renaissance Italy, like painting and architecture, which might influence Altaïr's view of beauty and design in building Assassin hideouts. And Altaïr could teach Ezio about the ancient Arabic philosophies that underpin the Assassin order, which could deepen Ezio's understanding of the Creed.
Agent 47 from the 'Hitman' series is extremely dangerous. He can blend into any environment, use a wide variety of weapons and methods to eliminate his targets without being detected. His cold - blooded and calculating nature makes him a top - tier assassin.
One great book on assassins in fiction is 'The Day of the Jackal' by Frederick Forsyth. It's a thrilling story about an attempt to assassinate the French president. The details of the assassin's planning and the cat - and - mouse game with the authorities are really engaging.