The product of boredom, here's something I made up.

Machinist

Abilities
01 Split Shot - Delivers an attack with a potency of 120.
02 Slug Shot - Delivers an attack with a potency of 180. Cast time: 1.5s (2.5s with Gauss Barrel).
04 Load - Loads your firearm with one round of special ammunition. Cooldown: 2.5s.
04 Lead Shot - Requires ammunition. Delivers an attack with a potency of 40. Additional effect: Deals damage over time with a potency of 40 for 21s.
06 Energy Field - Requires ammunition. Consumes up to 3 units of ammunition. Creates a damage absorption shield that negates damage equal to 5% of your max HP per ammunition unit loaded for 10s. Cooldown: 90s.
08 Heartbreak - Delivers an attack with a potency of 180. Can only be executed when target's HP is below 20%. Cooldown: 30s.
10 Leg Graze - Inflicts target with Heavy +40%. Duration: 12s. Cooldown: 20s. Ammunition effect: Heavy is replaced by Bind for 12s.
12 Reassemble - Makes the next shot a guaranteed critical hit. Cooldown: 90s.
15 Blank - Deals damage with a potency of 100. Additional Effect: 15-yalm knockback. Cooldown: 30s.
18 Grenado Shot - Delivers an attack with a potency of 120 to target and enemies near it.
22 Steady Shot - Delivers an attack with a potency of 100. Additional effect: Increases TP recovery rate by 10% for 30s. Cast time: 1.5s (2.5s with Gauss Barrel).
26 Hot Shot - Delivers an attack with a potency of 100. Additional effect: Increases the recovery speed of ammunition by 15% for 30s. Cast time: 1.5s (2.5s with Gauss Barrel).
30 Clean Shot - Delivers an attack with a potency of 200. Cast time: 2.5s (3.5s with Gauss Barrel). Ammunition effect: Removes cast time.
34 Head Graze - Delivers an attack with a potency of 50. Additional effect: Silence for 2s. Cooldown: 25s. Ammunition effect: Silence is replaced by Stun for 2s.
38 Rapid Fire - Reduces weaponskill recast timer to 1.5s and removes weaponskill cast times. Effect ends upon using 3 weaponskills. Duration: 15s. Cooldown: 90s.
42 Reload - Instantly recover all used ammunition units. Cooldown: 150s.
46 Dismantle - Lowers target's physical and magical damage dealt by 5%. Duration: 10s. Cooldown: 90s.
50 Gauss Barrel - Equips your current firearm with a gauss barrel, increasing damage dealt by shots with a cast time by 25%. While barrel is equipped, increases the cast time of certain shots by 1 second. Effect ends upon reuse.

Traits
08 Enhanced Dexterity
14 Lodged Lead - Extends Lead Shot duration to 30 seconds.
16 Enhanced Dexterity II
20 Increased Action Damage - Increases base action, drone and turret damage by 10%.
24 Enhanced Dexterity III
28 Enhanced Reassemble - Shortens Reassemble recast time to 60 seconds.
32 Expanded Reserves - Your aetherotransformer can now empower up to four units of ammunition.
36 Steady Focus - Steady Shot now has a 30% chance to refund its TP cost.
40 Engineered Advantage - Increases drone and turret damage by 20%. Enemies engaged by your turret or drone take 15% additional damage from your attacks.
44 Enhanced Hot Shot - Hot Shot now increases the recovery speed of ammunition by 20%.
48 Maximized Reserves - Your aetherotransformer can now empower up to five units of ammunition.


Job Abilities
30 Knight Drone - Deploys a close-combat drone that delivers auto-attacks with a potency of 80 to its master's target. Using a weaponskill will cause the drone to change targets. Cooldown: 10s. Shares cooldown with Rook Turret.
30 Retrieval - Retrieves a drone or autoturret from the battlefield. Cooldown: 10s.
35 Repair - Restores 15% of your drone or turret's max HP. Cooldown: 30s.
40 Rook Turret - Deploys a ranged autoturret that delivers auto-attacks with a potency of 100 to its master's target. Using a weaponskill will cause the turret to change targets. Shares cooldown with Knight Drone.
45 Promotion - Disables a drone or turret's attack mode. Knights will instead create a 30-yalm radius TP regeneration field. Rooks will create an MP regeneration field. Caster MP is slowly drained while drone or turret is promoted. Hypercharge Effect: Regeneration potency is doubled. Attack mode is reactivated upon reuse. Cooldown: 5s.
50 Hypercharge - Increases the output of your active drone or turret for 20s. Upon expiring, your drone or turret will overheat, becoming unable to act for 5s. Cooldown: 120s.
--Knight Drone: Doubles drone's attack power. Attacks dealt by the drone increase target's damage taken by 2% for 10s. This effects stacks up to 5 times.
--Rook Turret: Doubles turret's attack power. Attacks dealt by turret also deal damage to additional enemies within 5y or target.
52 Bishop Turret - Deploys a temporary autoturret that delivers auto-attacks with a potency of 50 to all nearby enemies. Duration: 20s. Cooldown: 45s.
54 Mana Bullet - Sacrifice MP to recover one unit of ammunition. Cooldown: 45s.
56 Penta-Detonator - Requires five ammunition units. Delivers an attack with a potency of 50. Places five explosive charges on target that detonate when target is attacked by your drone or turret. Explosion potency: 150. Cooldown: 90s.
58 Ricochet - Requires ammunition. Consumes up to 2 units of ammunition. Delivers an attack with a potency of 100(150) to target and 200(250) distributed among all nearby targets. Cooldown: 60s.
60 Combine - Requires Knight Drone or Rook Turret deployed. Combine your Knight Drone with your Rook and Bishop Turrets into a machina that delivers auto attacks with a potency of 200 to its master's target for 15s. Upon expiring, retrieves all drones and turrets and activates their respective cooldowns. If Bishop Turret is on cooldown, doubles the remaining amount of time until reuse. Cannot be used while Bishop Turret is active. Cooldown: 180s.


About Ammunition
Unlike what we currently have, ammunition in this build is to be used in set intervals. A pre-32 MCH has access to 3 units of ammo, and that gets expanded to 5 units by the time they hit lv48. Ammo can appear as a line of bullets under your MP bar, go "empty" when ammo consumed and slowly refill over time until it is "full", allowing you to use that unit of ammo again.

Ammo is consumed when you use attacks that require ammo after you've loaded them into your gun. Once an ammo unit is consumed, that unit recharges over 20 seconds. Ammo recharges one at a time, so if you use a skill that requires three ammo units, you'll have to wait a total of 60 seconds to have all three units back (the first unit coming back at 20 seconds, the second at 40 seconds, and the last one at 60 seconds).

Regardless of whether the skill is offensive (Ricochet) or defensive (Energy Field), you need to load each ammo unit into your gun separately for the skill to be usable. Unlike the current game, ammo does not have an expiration timer when loaded.

About Knight Drone
Knight Drones are one-wheeled machines that attack enemies in melee-range. They'd sort of look like this but with a little sword and shield instead of arms. They'd be a little hardier than the turrets, but still a far cry from egis or other pets.

Notes
- I've not taken the MCH story into account when writing this up. I know the story since I have MCH at 60, but this was more for fun and to also deal with my gripes with the job's current design.
- This build focuses a little more on gadgets and machines rather than gunplay.
- Ammunition is no longer a buff, but instead a resource that affects certain shots and is required for others.
- MCH starts out with 3 ammo units and traits expand that to a total of 5 by lv50.
- I wanted to give MCH a Spreader Module that would make all shots AoE while putting a 60% damage penalty on all attacks, but decided to go for Grenado Shot for now.
- Steady Shot's main bonus is the increased TP regeneration. The TP refund is more like an extra.
- Hot Shot's boost to ammo recovery should make it important enough to be used in the rotation.
- As for the "basic" shots, Split Shot is meant to be used on the move or when you need instant damage. Slug Shot is meant to be used when you can afford to stand still. Clean Shot serves a similar purpose but has the bonus of being instant with ammo, so you have to choose whether you can sacrifice your ammo, stand still for a caster clean shot or simply use another shot.
- Combining certain skills with ammo as the modifier (Leg Graze+Foot Graze, Head Graze+Suppressive Fire) seems like a good idea on paper.
- Gauss Barrel is on the chopping block, but I feel someone would complain if I didn't add it. This would probably feel better or make more sense if I were to give MCH the Spreader Module I originally wanted to give it.
- Bishop Turret is now a temporary pet that will be active for 20 seconds before withdrawing on its own.
- Mana Bullet was inspired by Blood Bullets from Bloodborne. Instead of sacrificing health, you sacrifice your entire MP bar to restore one ammo unit (the ability can't be used unless you're at 100% MP).
- Frankly speaking, I was never a fan of Wildfire, so Penta-Detonator is more sort of replacement.
- Yes, the idea for Combine came from Brute Justice. It's a no-brainer, IMO.
- Future plans would include more gun modules (maybe a laser attachment?), Pawn Drones that rush a target and explode after an interval and maybe the Rook Turret getting a combination with the MCH's gun to become an even bigger gun for a short time. While I'd love to eventually add a skill that makes your drones and turrets combine into a suit of power armor for you, that's a little too sci-fi for FFXIV.