A character like Zabuza could be a good antihero in an 'antihero Naruto crossover fanfic'. He was a mercenary who only cared about money and his own survival. In this fanfic, he could be brought back to life by some dark magic from another world. He then decides to use his skills and new - found powers to take over parts of the Naruto world. He would be a challenge for Naruto and the other ninja as he is a very skilled swordsman and has no qualms about killing. His antihero status would be enhanced as he goes against the values of the ninja villages.
Sasuke could be a good antihero in this kind of fanfic. He already has a dark side and a complex personality. He has his own goals and is willing to go against the traditional ninja ways to achieve them. In a crossover, he could be even more extreme, perhaps teaming up with some new, more malevolent characters from another world. He would still have his Sharingan powers which could be used in interesting ways against both Naruto and the new threats introduced in the crossover.