Genre and time period act as first-round filters of exclusion. Genre because ray guns aren't plausible in Contemporary Inspirational. Time period because Queen Elizabeth did not have a light saber (Walsingham probably would have peed with glee, but still).
Penchant and personality? The most important parts.
Everything a character wears from Underoos to a Claymore says something about the character. It also says something about their preferred environments and the social situations in which they often find themselves.
- Someone who wears a garrote disguised as a necklace? Overconfident Assassin. Requires close proximity and shadows. Has exceptional grooming so as not to give themselves away before the kill.
- Someone who carries a broadsword with a jeweled hilt? Noble Soldier. Requires open space and short timelines of actual conflict (like 5 mins or less). Accustomed to big, bold displays of aggression.
- Someone who wears a pendant filled with poison? Duplicitous Celebrity. Requires an atmosphere of distractions yet uninterrupted delivery of product. Is often seen yet not noticed.
- Someone carrying a Thales LithgowF90? Modern Day RAA Soldier on maneuvers. Requires intel, orders, and a war-torn environment. Functions best as one part of a whole. Team player.
Vadrigyn, the protagonist in my high fantasy LARCOUT? Venomous parasites that grow from her palms. Requires foe to be within reach. What does that choice of weapon say about her? That she's unaccustomed to a kind touch, to intimacy.
Her nemesis? Carries a cudgel but uses his magic to move stones to crush her. Requires area where rocks are present and for target to be within line of sight. What does it say about him? The unused cudgel represents the art of distraction and deception. The use of stone slabs to crush people? He has a penchant for excessive force.
With the choice of preferred weapon, you're also introducing weaknesses.
The solider with a sword? Can't use it in close quarters. Can easily be separated from it. The soldier using the automatic assault rifle? Easily spotted. Dependent on ammunition and proper function of multiple mechanical parts. Venomous parasites? Can't lay a hand on something without burning or killing it. Stone Mover? Loud noise. Big mess. Structure collapse.
Those who are adept at wielding their preferred weapon are also dependent on it, because repeated use--that whole "the weapon is an extension of self" belief--has created a series of habits during defense or aggression. Those habits can (and should) be used against the beloved character(s).
Yeah but MAGIC...
Creating a unique magic system for your world and characters is awesome and fun...and still bound by a logical give and take. It still needs to be a dance of strengths and weaknesses. It still needs to show balance within the character and within the world. "All Magic comes at a cost..." because it needs to to make good storytelling. Otherwise willynilly unbridled unchecked use means there's no character growth. No character growth means it's a boringass story. So, when constructing a new world of magic and assigning who can do what, ask if the magic enhances the character through:
- Describing the character's station/class
- Reflecting the character's environment
- Caging/hampering the character with dependencies
- Exposing unique habits
- Succumbing to assorted weaknesses borne of wielding said magic.
So, dear readers, when next you think of putting a flail, a Beretta, or an amulet in your characters' hands, consider what that weapon is really saying about your characters.