Another interesting OC for Samurai Jack fanfiction could be a young apprentice that Jack takes under his wing. This apprentice could start off inexperienced but grow throughout the story. They could come from a village that was recently attacked by Aku's minions, and their journey of revenge and growth while learning from Jack would make for a great fanfiction.
An OC who is a magical being from a different dimension could also be a great addition to Samurai Jack fanfiction. This being could have the ability to manipulate time, which would be very useful in Jack's quest. They could be initially neutral but be convinced by Jack's noble cause to use their powers to help him defeat Aku.
One idea could be creating an OC who is a long - lost relative of Samurai Jack. Maybe a cousin who has also been trained in the ways of the samurai and comes to help Jack in his fight against Aku. This OC could have unique abilities, like the power to control the elements, which would add new and exciting elements to the story.