Unpopular opinion here, but I want Machinist to be more of a Machinist than a Gunner. Like, their Gun is their main arm, but let's not forget the roots of Machinists in the FF universe.
FF4: Cid wields hammers in battle and can use his special skill Analyze to examine enemy stats.
FF4 TAY: Luca is also able to use Analyze and has Big Throw, which lets her throw her weapon like a boomerang at an enemy so that she does full damage from the back row.
FF6: Edgar's job was Engineer, but the Game Boy Advance version renamed the job title to Machinist. He equips spears and can use the Tools command to attack enemies with various mechanical gadgets. His Tools include: Auto-Crossbow, Bioblaster, Chainsaw, Drill, Air Anchor, Flash, Noiseblaster, Debilitator.
FF7: Cid Highwind is a combination of Machinist/Dragoon. While he does wield a spear and uses jump attacks, he also throws explosives, and commands the Airship - Highwind to unleash a volley of missiles at his enemies. Additionally, Barret is somewhat of a Gunner/Machinist hybrid, using Grenades, Satellite Beams, and heavy artillery.
FFT: Mustadio is the most commonly known Machinist. Originally translated as "Engineer", but later renamed Machinist in the WoL version. He wields a handgun and debilitates enemies with aimed shots.
FFXI: Puppetmaster is the FFXI equivalent of Machnist. While the PUP itself doesn't have many abilities, they command a robot that uses attachments which are more or less gadget/tools (Flashbulb, Heatsink, Tension Spring etc).
That said, I'd like for 5.0 MCH to focus less on fancy gun actions and more on using complex machines and gadgetry. Maybe their Turret builds up 2-3 points of "Ingenuity" per auto-attack and at 80+, the MCH can use "Promotion" which transforms the Turret into a mecha like Rival Wings (King Turret or Queen Turret depending on if you promote Rook or Bishop). Basically, in this form, you have 2-3 pet actions to use in a 20s duration and then end it with a finishing blast. Like the non-promoted form, King would specialize in ST and Queen would specialize in AoE. Would kinda be like Summon Demi Bahamut, but for MCH.
Hell, what if we summon a Mustadio replica and he comes down and whips out his Sniper Rifle and does some cool attack for the level 80 skill?