I have this idea for an artificer subclass I'd want to make, but not a clue on how to make the abilities or features. It's like, a sort of jekyl/hyde or monstrous theme, a mad-scientist that makes formulas that turns other or themselves into monsters. Maybe not "monsters" from dnd persay, but a monstrous form that lets them become a powerful melee combatant with maybe some RP or mechanical drawbacks. I'm thinking kind of close to the class features from the bloodhunter lycan subclass. Anyone got some advice for materializing your subclass ideas. Been a few years since I've made one. Just haven't been feeling it lately until now.

Here is an idea to get you on your feet. Maybe try and look at other people’s subclasses for inspiration? I will give you this idea only as the subclass should be your OWN. Make it yours with your own personal touch.

Starting at 3rd level, you have the power to create anything. As an action, you can spend 10 minutes tinkering and creating a weapon of your choice. Each weapon has a component that is required, and modifications may be made at the DM’s discretion. 

Bow or Crossbow: A wood or metal rod, and a piece of string or a sturdy, elastic resource

Swords, Daggers, knifes: A metal sheet, or 500 GP, SP, or PP

Firearms: A metal sheet, or 500 GP, SP, or PP, and a pinch of gunpowder

Armour and Shields: 1000 GP worth of GP, SP, or Leather (Depends on the type of armour being made. PP = Heavy Armour, SP = Medium Armour, Leather strips = Light Armour)

You are proficient with your creation while you wield it. In addition, you gain proficiency in alchemist’s tools.

I combined Alchemist Artificer with Beast Barbarian to get the below:

Tool Proficiency

3rd-level feature

You gain proficiency with alchemist’s supplies. If you already have this proficiency, you gain proficiency with one other type of artisan’s tools of your choice.

Mutagen Spells

3rd level : Ray of Sickness, Arms of Hadar

5th level : Melf's acid arrow, Dragon's Breath

9th level : Vampiric Touch, Bestow Curse

13th level : Polymorph, Blight

17th level : Cloudkill, Arcane Hand

Experimental Mutagen

3rd-level feature

Whenever you finish a long rest, you can magically produce an experimental mutagen in an empty flask you touch. Choose an effect from the list below for the elixir’s effect, which is triggered when someone drinks the elixir. As a bonus action, you can drink the elixir. You can create additional experimental mutagens by expending a spell slot of 1st level or higher for each one. When you do so, you use your action to create the elixir in an empty flask you tou

Creating an experimental mutagen requires you to have alchemist’s supplies on your person, and any elixir you create with this feature lasts until it is drunk or until the end of your next long rest. When you reach certain levels in this class, you can make more elixirs at the end of a long rest: two at 6th level and three at 15th level. Roll for each elixir’s effect separately. Each elixir requires its own flask.

• Long Legs : the drinker's movement speed is increased by 10 ft for 1 hour
• Sensitive Nose : the drinker gains advantage on Wisdom (Perception checks) that rely on smell until they finish a long rest.
• Muscles : the drinker's Strength score is replaced by your Intelligence score for 1 minute.
• Claws : Each of the drinker's hands transforms into a claw, which can be used as a weapon if it’s empty. It deals 1d6 slashing damage on a hit. Once on each their turns when they attack with a claw using the Attack action, they can make one additional claw attack as part of the same action. These claws last for 10 minutes.
• Tentacle/Tail. The drinker' grows large tentacle or tail that can be used to hold objects up to 10 lbs , it cannot wield a weapon even if it can hold it, or grapple a creature. The tail / tentacle lasts until they finish a long rest.
• Jaws. Your mouth transforms into a bestial muzzle or great mandibles (your choice). It deals 1d8 piercing damage on a hit. Once on each of your turns when you damage a creature with this bite, you regain a number of hit points equal to your proficiency bonus, provided you have less than half your hit points when you hit.
• Gills. The drinker can breathe underwater for 1 hour.

A creature has Disadvantage on Charisma checks while under the effects of your mutagens.

Extra Attack
5th level

You can attack twice, rather than once, whenever you take the Attack action on your turn.

Potent Mutagens
5th level

You can choose two effects from the list whenever you create an Experimental Mutagen.

Fortified Mutagens
9th level

Other creatures other than you can safely consume your mutagens. Whenever a creature drinks a mutagen they gain 2d6 + your Intelligence modifier temporary hit points.

Second Life
15th level

When a creature is reduced to 0 hit points while under the effect of one of your mutagens, it instead is reduced to 1 hit point and it loses all benefits of your mutagen.

Quote from Agilemind >>

I combined Alchemist Artificer with Beast Barbarian to get the below:

 

Tool Proficiency

3rd-level feature

You gain proficiency with alchemist’s supplies. If you already have this proficiency, you gain proficiency with one other type of artisan’s tools of your choice.

Mutagen Spells

3rd level : Ray of Sickness, Arms of Hadar

5th level : Melf's acid arrow, Dragon's Breath

9th level : Vampiric Touch, Bestow Curse

13th level : Polymorph, Blight

17th level : Cloudkill, Arcane Hand

Experimental Mutagen

3rd-level feature

Whenever you finish a long rest, you can magically produce an experimental mutagen in an empty flask you touch. Choose an effect from the list below for the elixir’s effect, which is triggered when someone drinks the elixir. As a bonus action, you can drink the elixir. You can create additional experimental mutagens by expending a spell slot of 1st level or higher for each one. When you do so, you use your action to create the elixir in an empty flask you tou

Creating an experimental mutagen requires you to have alchemist’s supplies on your person, and any elixir you create with this feature lasts until it is drunk or until the end of your next long rest. When you reach certain levels in this class, you can make more elixirs at the end of a long rest: two at 6th level and three at 15th level. Roll for each elixir’s effect separately. Each elixir requires its own flask.

• Long Legs : the drinker's movement speed is increased by 10 ft for 1 hour
• Sensitive Nose : the drinker gains advantage on Wisdom (Perception checks) that rely on smell until they finish a long rest.
• Muscles : the drinker's Strength score is replaced by your Intelligence score for 1 minute.
• Claws : Each of the drinker's hands transforms into a claw, which can be used as a weapon if it’s empty. It deals 1d6 slashing damage on a hit. Once on each their turns when they attack with a claw using the Attack action, they can make one additional claw attack as part of the same action. These claws last for 10 minutes.
• Tentacle/Tail. The drinker' grows large tentacle or tail that can be used to hold objects up to 10 lbs , it cannot wield a weapon even if it can hold it, or grapple a creature. The tail / tentacle lasts until they finish a long rest.
• Jaws. Your mouth transforms into a bestial muzzle or great mandibles (your choice). It deals 1d8 piercing damage on a hit. Once on each of your turns when you damage a creature with this bite, you regain a number of hit points equal to your proficiency bonus, provided you have less than half your hit points when you hit.
• Gills. The drinker can breathe underwater for 1 hour.

A creature has Disadvantage on Charisma checks while under the effects of your mutagens.

Extra Attack
5th level

You can attack twice, rather than once, whenever you take the Attack action on your turn.

Potent Mutagens
5th level

You can choose two effects from the list whenever you create an Experimental Mutagen.

Fortified Mutagens
9th level

Other creatures other than you can safely consume your mutagens. Whenever a creature drinks a mutagen they gain 2d6 + your Intelligence modifier temporary hit points.

Second Life
15th level

When a creature is reduced to 0 hit points while under the effect of one of your mutagens, it instead is reduced to 1 hit point and it loses all benefits of your mutagen.

 

 

 
That's pretty good. The only thing I would change (out of personal taste), is that I would allow for more durable mutations that use up infusion slots, just to give it some connection to the base class.

One issue with this is that you're dealing with conflicting roles and class identity.

If the job of this subclass is to become a hulking brute, is that really an artificer? Is it using any artificer class features at all, or is it just that artificer has mad scientist vibes?

When I start working on a subclass and realize that it's built entirely on flavor, I step back and reconsider if I'm in the right class. Yes, an artificer is kind of like a mad scientist, but in D&D terms an artificer is a half-caster that specializes in magic items. An artificer subclass should feed into that, elaborate on that theme and be reinforced by it. I don't think this does that. In fact, it pushes against it. A hulking brute isn't going to cast spells. Swinging with your claws means you can't use your infused weapon.

I do think it's a cool idea, and it's an itch that Blood Hunter doesn't manage to fully scratch. But this feels like the wrong approach to me.

That's what I was worried about. So far, the closest I have is a character idea split between beast barbarian and alchemist artificer, flavored as a mad scientist that can transform themselves into a monstrous form. Artificers never seem to really feel like a class that lends itself to a larger number of subclasses. At least for me.