It could be an original character. For example, a ninja who was born into a minor clan but had great potential. However, due to political intrigue and jealousy from a major clan, he was framed and banished. He then became a mercenary to survive. Another option could be a character who was once a part of a secret ninja organization and was banished when he disagreed with their methods. Now, as a mercenary, he returns to the Naruto world with a vendetta against those who wronged him.
It could be an original character created by the fanfic author. This character might be a skilled fighter, maybe with a unique weapon or fighting style, like using a special kind of kunai combined with lightning - based chakra attacks.
For the banished mercenary, his character development can be made interesting by exploring his backstory. If he was banished because of a misunderstanding, gradually revealing the truth behind it can be a great way to develop his character. He might initially be a lone wolf, but as he gets involved in various adventures, he realizes the importance of having comrades. His views on the shinobi way could change over time. For instance, he might start off believing that only power matters but then, after seeing the selfless acts of others, he begins to understand the value of things like loyalty and friendship. This transformation from a jaded mercenary to someone with a more positive outlook on the shinobi world would be very engaging.
It could be an original character. For instance, a ninja who was wrongly framed for a crime and banished. They would then be the focus as they navigate their new life as an assassin.
The main antagonist could be an old - school ninja who believes in the traditional and often harsh ways of the ANBU. They see Naruto's actions as weakening the village and set out to oppose him at every turn, trying to bring back the old ANBU regime.
Another option could be Kakashi. Suppose he is banished because of some incident related to his Sharingan. He has so much history and experience, and seeing him outside of the village structure, relying only on his Sharingan and his wits, would make for a great story. He could also have flashbacks to his past experiences with the Sharingan while dealing with the challenges of being banished.
Danzo might be the main villain. He's known for his underhanded tactics. He could have banished Naruto to create chaos and then sent his Root agents to destroy Konoga for some political or power - related reasons. He might see Konoga as a threat to his control over the village in some way.
One idea could be that the banished mercenary has unique jutsu skills learned from different lands during his mercenary days. He could return to the Naruto world with a new perspective and hidden motives, perhaps trying to unite different ninja clans against a new, greater threat. Another plot idea is that he has a connection to an ancient prophecy that was thought to be forgotten, and his banishment was actually part of a grand scheme to protect him until the time was right.
It could be the very people who banished Naruto in the first place. They might fear his new - found power and try to stop him from returning to the village or from getting revenge. They would likely use underhanded tactics to try and take him down.
Perhaps Danzo could be the main antagonist. He was always a bit of a shadowy figure in the original Naruto series. In this fanfic, he might see Naruto and Sasuke as a threat to his own control over Konoha. So, he uses his influence to have them banished. He could be secretly working with external forces to further his own agenda while keeping Naruto and Sasuke away from Konoha.
A grizzled veteran mercenary who has seen many battles and has a lot of combat experience. He could be cynical at first but slowly warm up to the RWBY team.