Heyo! I've been working on a lot of ideas for D&D lately, and I wanted to share my latest one. I really love the idea of the Artillerist subclass for the Artificer, but it feels very much like "haha extra damage go brrr" is the only thing it provides in terms of player engagement. Just plop your Eldritch Cannon down and you've got free damage for your bonus action. It just doesn't feel very involved or interesting. So I decided to take a leaf from two of my favourite video games and tossed this together! Those games being TF2 and Deep Rock Galactic. Read on, and please do give feedback! I wanna make this as balanced as I can! 

The Gunslinger (requires attunement by an Artificer with the Artillerist subclass):
Built by Dell Conagher with the guidance of notes and diagrams from past Conaghers, the Gunslinger is an incredible feat of both mechanical and magical craftsmanship. The Gunslinger is a prosthetic that replaces your right arm, starting halfway down the forearm. It is similar in appearance to its form in Team Fortress 2, if not completely identical. It functions like the Prosthetic Limb found in Tasha's Cauldron of Everything, with the following features:

Handyman: The Gunslinger can function as a set of tinker’s tools. Since you are an Artificer, you can use it as an arcane focus, and it’s eligible for the Arcane Firearm feature. Standard rules for arcane focuses still apply. The Gunslinger has a number of charges equal to a third of your Artificer level (rounded up), and they recharge fully after a long rest. You gain the following options:

• Quick Fix: If one of your Eldritch Cannons takes damage that would destroy it, you can use your reaction to move up to half of your speed towards that Cannon. If you are within 5 feet of it at the end of this movement, you can expend a charge as part of the same reaction to restore hit points to the Cannon equal to 3 times your Artificer level.
• Turret Whip: While within melee range of your Cannon, you can expend a charge as a bonus action to deal 1d8 bludgeoning damage to your Cannon. This damage cannot reduce the Cannon below 1 HP, but it also cannot be prevented. In exchange, the next time your Cannon is activated, it gains a damage bonus equal to your Intelligence modifier, and the number of dice rolled for its effect are doubled. The number of damaging d8’s rolled increases at 9th level (2d8) and 15th level (3d8).
• Frontier Justice: If one of your Cannons that has dealt damage to an enemy or or has been hit by Turret Whip but not benefited from it yet is destroyed, you can expend a charge to gain advantage on your next attack roll against its destroyer, and this attack is a critical hit on a roll of 18-20.

This seems like a lot for what I called a sidegrade, and it might be. But that's why I've come here, to see what you guys can suggest for it! The design philosophy behind this variant of the Gunslinger is pretty simple but ideally changes how you use your Eldritch Cannon. Instead of just letting your Cannon sit there and shoot, you gain 3 extra options: the ability to increase effectiveness on your next turn, the ability to potentially save a Cannon from death, and the ability to capitalise on a Cannon's destruction by punishing the scoundrel who dared to hurt your precious machinery!

This should probably go in the Homebrew forum.

Good point, I just wanted to pop it here to see what other DMs would think of it

Without a low HP turret like the mini- sentry or the ability to deploy multiple support constructs to the field (teleporters, dispensers) like you can in TF2, I'm not sure how effective the frontier justice would be. The likelihood of a cannon going down and providing a charge is too low and a far between. It makes sense for TF2 because the gameplay is so much faster.

I think you would need a new type of low cost cannon. While it might not deal as much damage as one of the standard cannons, it could be summoned multiple times in a battle without requiring a spell slot or long rest.