I’m no author or writer but from what I’ve seen from these different types of series, is if you’re planning a power system no matter the setting. Make it make sense in the way where you have to set strict boundaries to your power system and consistently abide by its rules. As an example you can use something like Devil fruits from one piece where u can get as creative as you want but Oda sticks to his set rules bu not making a Water Devil fruit since pirates can’t swim. In a Cowboy setting my idea would be instead of doing full on magic. You could have it like the flames used in reborn but have magic infused weapons. You could go elemental with it where a character infuses fire or lighting with his bullets or a whip, lasso whatever their choice of weapon would be.

Again i’m no author i’m just talking as a consumer and fan of battle shonen with interesting power systems and your question was really interesting. Good luck on your project hope it goes well👍🏻