Hey! You can absolutely go off-piste with the character options and fill in the blanks with your own ideas. However, the further you stray from the tone of the suggestions, the harder you might find it to fit whatever you come up with into the broader story.
For example, if you ended up with:
“I wear mechanised armour and ammo belts and carry a giant machine gun with me pretty much everywhere.”
It might make some of the framing around your journey a little harder to square! TBH even something as extreme as that would probably still work fine though—it definitely won’t ‘break’ anything. If, as I expect, your ideas are closer to the witchy norms established by the book, I’m 100% sure you’ll be fine :)