A cool 'fear the walking dead crossover fanfiction' idea would be a crossover with 'Stranger Things'. The Upside Down could be connected to the zombie outbreak in some way. The kids from Hawkins could team up with the survivors of 'Fear the Walking Dead' to fight both the zombies and the strange creatures from the Upside Down. It would be interesting to see how their different skills and knowledge would work together.
There's also 'The Unseen Heroes' which is a fanfiction that explores the stories of the background characters in 'Fear the Walking Dead'. It's fascinating as it shows how these often - overlooked characters can have important roles in the overall apocalyptic world of the show.
Morgan from 'Fear the Walking Dead' and Rick from 'The Walking Dead' would be an interesting pairing. Morgan has his unique philosophy on life and death, and Rick is a natural leader. Their different leadership styles could either clash or complement each other in a crossover scenario.
First, know the source material well. Understand the characters, their motives, and the world of 'Fear the Walking Dead'. For example, if you're writing about Morgan, you should be aware of his backstory and his journey in the show.