Menu-based RPGs often include features that crudely simulate conditions in melee combat. Events like dodging, blocking, counterattacking etc. all have a rough analog in swordfights or fistfights where posture and momentum and reactions have an effect on the outcome of an encounter. Gunfights work differently, since nobody parries or dodges a bullet, and firing a rifle doesn't throw you off-balance enough to make a riposte more effective against you. Instead, elements like stance, focus, situational awareness, fields of fire, zones of cover, rate of fire and the ever-dramatic pause to reload would factor into the equation for success.
There are games that do a good job of presenting gunfights in a turn-based RPG, like Silent Storm, but their melee systems are generally pretty basic by comparison and all the best ones use spatial relationships as part of the game space. A concealed sniper taking a turn to zero in and control his breathing before taking a shot on a distant target is like a mage channeling mana or a spearman taking a rooted defense stance, so the mechanics aren't foreign, they're just different. Quick-draw moves, target transitions, hip-fire versus aimed fire, all of this can be worked into a successful shooting RPG system.
Some trouble might arise when you blend the shooting and the stabbing together, since melee characters and firearm characters will essentially be playing by different rules. Knife to a gunfight and all that. There's the old 21-foot rule, which states that the average knife-wielding assailant can cover seven yards of distance and stab you in the neck faster than you can draw a pistol and shoot him. It's a point of focus for police trainers when suspect interview comes up, and it's why you see the police on Cops cuffing dudes "for our safety, sir" even though the "suspect" is just a random elderly hobo who may or may not have shown his pecker to a passing motorist. Nobody wants to get stabbed, and it's way easier to stab a cop than it is to hire and train one, so rules get made.
So you can achieve balance that way, especially if you have the creative power to make guns cumbersome or have them preclude certain armor types. You could make gunslingers invincible badasses until their ammo runs out, so they can mow down knights and wizards for a turn or two, then while they're shoving new cartridges into their revolver some barbarian strolls over and shoves their revolver down their throat. Guns could be terribly slow, so your musketeers can lay down somewhat respectable fire, one shot every three turns or so, until the enemy closes on them, and then you either fix bayonets or reach for your rapier, because it's all hand-to-hand from here on out.
