I developed this subclass for as player in my Catamaran game who wanted to play a more sciencey rogue.  He was  the island alchemist and astronomer and none of the existing subclasses at the time really fit what he wanted to do.  He had a flintlock pistol, which was super rare in the setting, so a lot of the abilities for this subclass were built around him using his gun. This subclass has been playtest from levels 3-12 and played really well.

Gadgeteer

You use learning and observation to your advantage, taking the materials of the natural world and bending them to your will through science.  Others may see your skills as being akin to magic, but your work is not based on any power arcane or divine.  Instead you use chemistry, physics, and other fields of study left ignored by most, finding power in rarely held knowledge.  

Master of Science

At third level you gain proficiency with the alchemist’s and tinker’s kit.  If you are already proficient in these kits, you may apply double your proficiency bonus when using them.  

Gadgets

When you choose this archetype at 3rd level, you learn gadgets that are fueled by special dice called cunning dice. You learn three gadgets of your choice, which are detailed under “Gadgets” below. Using a gadget requires an action, but the specific type of action varies from gadget to gadget.  

You learn two additional gadgets of your choice at 9th, 13th, and 17th level. Each time you learn new gadgets, you can also replace one gadget you know with a different one. 

Cunning Dice. 

You have four cunning dice, which are d8s. A cunning die is expended when you use it. You regain all of your expended cunning dice when you finish a short or long rest. You gain another cunning die at 9th level and one more at 17th level. 

Saving Throws. 

Some of your gadgets require your target to make a saving throw to resist the gadget’s effects. The saving throw DC is calculated as follows: Gadget save DC = 8 + your proficiency bonus + your Intelligence modifier.

Thingamabobs

Beginning at 3rd level you gain the ability to use an action to make small simple gadgets out of the materials you carry around with you.  You may create any of the following effects and they each last four hours. You may use this ability number of times equal to your Proficiency bonus and these uses reset after a long rest  

• A light source that generates bright light for 10 feet and dim light for 10 feet beyond that.
• Any set of tools.  
• A heating element that grants the user advantage on saves against cold weather.
• A loud fireworks display that can be seen and heard for 500 feet, though this one only lasts one minute.

Invention

At 9th level, you have developed a unique invention that does what others can only do through magic.  Choose one spell of 2nd level or lower from any spell list; you can recreate that spell’s effect through science twice per long rest with a spell level equal to your Intelligence bonus (minimum 2).  The selected spell is considered to only have a somatic component, thus making you immune to silence, and this ability is not affected by anti-magic fields or similar effects.  You should work with your DM to select a spell that makes sense and to craft the fiction that explains how it works, such as home brew healing potions for cure wounds spells or some manner of explosive for thunderwave.  You may change your selected spell during any long rest.

Scientific Breakthrough

At 13th level your cunning die increases in size to d10s.  At 17th level they increase to d12s and if you have no cunning die when you roll initiative, you gain one cunning die.  

Gadgets

Antidote

You use some general poison antidotes to help an adjacent ally fight off the effects of poison.  As a bonus action you can give one of your cunning dice to the ally you assist and they can add it to a saving throw against any manner of poison made before the end of their next turn.  

Armor Piercing Shot

When you use an Attack action, you can spend a cunning die to hit with a specially devised weapon, adding your cunning die to your attack roll. You make this choice after the dice are rolled, but before the results are revealed.   

Bola Attack

When you hit with an Attack action, you can spend a cunning die to hit with a specially designed weapon, wrapping the target of your attack up in rope or net, restraining it until freed. This modification has no effect on creatures that are incorporeal or amorphous, or creatures that are Huge or larger. A restrained creature can use its action to make a DC 10 Strength check, freeing itself on a success. If a creature attempts to free itself in this fashion, it suffers damage slashing equal to your cunning die for each attempt. Dealing 5 slashing damage to the bola (AC 16) also frees the creature but inflicts damage equal to a roll of your cunning dice on the target, ending the effect.

Caltrop Attack

When you hit with an Attack action, you can spend cunning die to use a weapon that releases caltrops in a five foot radius on the ground around the target. Any creature passing through this area must make a Dexterity save or suffer piercing damage equal to your cunning die, though they may treat the affected area as difficult terrain in exchange for getting advantage on the Dexterity save. Each time a creature makes a save against your caltrops roll your cunning die; if you roll a 1 the caltrops are scattered and no longer effective. 

Explosive Shot

When you hit with an Attack action using a ranged attack, you can spend a cunning die to hit with an arrow or bullet equipped with an explosive head. Creatures within 5 feet of the target of the attack take fire damage equal to a roll of your cunning die. 

Fire Attack

When you hit with an Attack action, you can spend a cunning die to hit with an weapon carrying a container of oil and a flammable head, lighting the target on fire. The target takes fire damage equal to your cunning die at the beginning of its turn. This occurs each round until the target succeeds at a Dexterity saving throw at the end of its turn or it uses an action to douse the flames. 

Glue Attack

When you hit with an Attack action, you can spend a cunning die to hit a Large size or smaller  creature with a weapon fitted with an alchemical glue canister, covering the target in glue. The target’s speed is reduced to 0 until it spends an action breaking free or it succeeds at a Strength saving throw at the end of its turn. Each round the target is stuck in the glue it takes poison damage equal to your cunning die at the beginning of the target’s turn. 

Grapple Arrow

When you hit with an Attack action against a target at a higher elevation than you, you can spend a cunning die to hit with an arrow with a specially made grappling hook arrowhead and pulley system, allowing you to quickly winch yourself up to wherever your grappling arrow lands. You gain a Fly speed equal to double your Speed for your turn, but you must end the round within five feet of the location the arrow struck. You may add your cunning die to your armor class until the beginning  of your next turn.

Illuminating Arrow

When you hit with an Attack action, you can spend a cunning die to hit with an alchemically treated arrow or bullet that erupts into a shining beacon of light, illuminating the space around where it lands. The arrow produces bright light for a twenty foot radius and dim light for twenty feet beyond that.  At the end of each of your turns roll your cunning die; if you roll a 1, the arrow burns out and the light ends.  

Knockback Arrow

When you hit with an Attack action, you can spend a cunning die to hit with a specially designed explosive weapon that strikes the target with magnified force. The target suffers additional thunder damage equal to your cunning die and must make a Strength saving throw or be moved 10 feet away from you.

Lightning Arrow

When you hit with an Attack action, you can spend a cunning die to hit with an weapon outfitted with a battery and lightning conductor, sending an electrical jolt through the target. The target takes lightning damage equal to your cunning die and must make a Constitution saving throw  or be stunned until the end of its next turn. 

Material Arrow

When you hit with an Attack action, you can spend a cunning die to hit with a weapon tipped with a special substance or material, such as cold iron or silver. It cannot mimic an energy type or anything without mass. The arrow will interact with any resistances and immunities of the target as appropriate for that material, and the target takes additional piercing damage equal to your cunning die from the attack. 

Poison Strike

When you hit with an Attack action, you can spend a cunning die to hit with a weapon that is coated in poison. You add your cunning die as additional poison damage to the attack. The target must also make a Constitution save or become poisoned; the target can repeat the saving throw at the end of each of their turns to remove the poisoned condition.  

Smelling Salts

You use some smelling salts to help an adjacent ally regain their senses.  As a bonus action, you can give one of your cunning dice to the ally you assist and they can add it to a saving throw to break free of any manner of charm or sleep effect made on their next turn.  

Smoke Arrow

When you hit with an Attack action, you can spend a cunning die to hit with an arrow or bullet containing an alchemical smokestick, releasing a cloud of smoke ten feet in diameter. The area becomes heavily obscured. At the end of each of your turns roll your cunning die; if you roll a 1 the cloud dissipates. Also any wind stronger than 10 miles per hour will disperse the cloud.

Stimulant

You dose an ally with a stimulant that reduces pain and increases alertness.  As a bonus action, you grant an adjacent ally temporary hit points equal to the amount rolled on your cunning die and a bonus to all Perception checks equal to your cunning die as long as those hit points remain.  These hit points fade after a short rest.  

Thunder Shot

When you hit with an Attack action, you can spend a cunning die to hit with an arrow or bullet equipped with a special arrowhead that releases a clap of thunder when it strikes. The target takes additional thunder damage equal to your cunning die and all creatures within a 10 feet radius must make a Constitution save or be deafened. The targets can repeat the Constitution saving throw at the end of each of their turns to regain their hearing. 

Weak Point

As a bonus action you can point out the weak points of creatures that have physical forms to an ally, granting that ally a bonus to an attack roll made during their next turn equal to your cunning die and allowing their weapon to count as magical for that attack.