Faveryy
Here a few ideas: Level system with xp that you gain with missions killing or learning stuff An inventory to put the weapons ammunition food etc... Powers but not too op like night vision predict the next 3sec or likewise more op may be limited like 2times a day etc... You can introduced already existing characters or new ones. I have seen all the movies and played a few games but I forget a lot so introductions are nice and trie to create more profound characters as it will improve the novel also the interactions make it all more lively. I think its a really nice idear for a fanfic so good job. PD cold weapons are really handy so making him learn how to use swords knife arrows will be a good idea.
Good powers would just be plain ol' superhuman strength, speed and reflexes. He's gonna need it with some of the monsters he's gotta go up against. You could add some martial techniques from different anime/manga as well. Just don't make him too overpowered by giving him a ****ty sharingan or something similar.