[Snake]

Aaaaaa. I just recalled typing this somewhere so I'm putting this again. Guns could use a big rework at this point if we're aiming for innovations. But those are mere suggestions, nothing I'm actively seeking to convince to be implemented:

Guns could act differently by their type alone.

All Guns - Damage's split on SKI/GUI, 90% SWA is the minimum they get, 120% is the maximum. All guns also will receive far-shot penalties in double or triple (probably lowered by a talent or trait).

"Reload" should be for both guns, regardless if they're main hand or off-hand, it will reset both 'ammo' counts to full.

Launchers:
- 6M to reload.
- Can attack only one time before needing a reload. (Launcher's gimmick, strong but slow)
- Make the damage work like Lantern Bearer, higher damage inflicted the closer to the center of where you've targeted is. (max. 300%, min. 10%)

Shotguns:
- 4M to reload.
- Can attack one/two times before needing a reload (depends on the gun, but is supposed to be like a faster Launcher).
- 4 range triangle AoE by default, with increased critical damage the closer the enemy is (50% damage per square, max of 200%).

Handguns:
- 2M to reload.
- Can attack two/three times before needing a reload (depends on the gun).
- Always ignores 50% of the enemy's DEF/Armor.

Rifles:
- 3M to reload.
- Can attack one/three times before needing a reload (depends on the gun).
- Higher accuracy the farther the enemy is from your position.

Now!

As of why I think WIL shouldn't be really the decisive factor: WIL becoming now mandatory to increase a gun's damage will only make Magic Gunners do more damage and become more tanky against Physical Damage aside of increasing how many Overcharges and skills they can spam freely due to their enormous FP pool, instead of increasing the use/damage of those in general, which is probably the only reason we're complaining that 'guns suck'. We want guns to be acceptable without Magic Gunner, so other classes can use them too. (I actually love how Daggers work, 'VAs are amazing at using them, but their GUI scaling allows them to have their own originality and use for other classes'. Guns could use this treatment.)

Try adding GUI to this instead of WIL. WIL's role is to give Magic more damage, and it doesn't quite benefit any other types of weapon, making wielding them with a melee weapon by the side in some class cases (Hi, Demon Hunter! Hi, Engineer!) kind of terrible to pull off.
Sure, WIL can give some extra FP, Elemental ATK and slots, but there's not a single good WIL weapon to pair up with them aside tomes and Magic. GUI can easily be the best alternative for guns.
As for why GUI? Higher critical damage on guns sure won't be that much of a balance tip, since it requires both hitting AND going through the enemy's critical evade on a RNG, aside that, guns naturally have a terrible critical chance.

TL;DR, GUI instead of WIL, as MG shouldn't be really getting a buff from this. We want guns to be used effectively by other classes other than only MG/Mage Promos.
Guile is a more optimal choice than Willpower, stat wise and what it does.