I created a subclass for Sorcerer, based on fire: Fire Soul. And I tend to struggle with balance so I would like feedback as I am not in a position to test this myself in a game. I fear the capstone Master of Flames is a bit strong granting 3 things, but those things individually aren't strong on their own for such a level and they have limitations and high sorc point cost.

The idea I'm going for is somebody who is born to the fire, the aspect of nature - the fire - is within them, and so this person should never fear fire or be inconvenienced by it, it's part of them, it would be like fearing your own hand. Enemy mage casting fireball? Oh, no, that's not going to work. Or somebody who could live in a volcano or on the elemental plane of fire as easily as commonfolk live in their safe and comfy houses. The ultimate expression of this is turning into fire and creating living flame.

But such a thing is difficult to scale into levels and be balanced in a game world. So, I'm hoping for feedback in improving this.

My expertise on sorcerers is limited, but I'll run down your list of features from my perspective.

• Sparks & Embers: Thematically on point. The cantrips are reasonable. Level 1 fire spell choice is creative.
• One with fire: What level do they get this at? Fire resistance is useful, but generally not overpowered. Immunity to your own fire spells is cool. Just begging to drop a fireball on yourself and come walking out of the flames.
• Fire's wrath: You are going to get a lot of mileage out of this because fire resistance is one of the most common creature resistances. Doesn't seem overpowered, but extremely useful.
• One with fire (improved): Complete immunity to fire damage. It's level 12, so it should be fairly powerful, which it is. Just ask yourself how you could abuse complete immunity. It might be fine, but definitely worth dwelling on for a while. If you decide it's a little too powerful, maybe make it a duration thing you can use once per short/long rest or per day. I don't know. Maybe I'm being overly cautious with this one and it's no big deal.
• Fire safety: Solid choice. It works a type of fire-specific careful metamagic into the ability, which is appropriate for a sorcerer.
• Master of flames: It's powerful. It's practical. I like it. I'm not a good judge of whether it's too powerful, but I will say that any of the three seem reasonable by themselves. Maybe all three are ok too. I'll leave that to someone who knows sorcerers better than I do.

A few general thoughts: All of these powers are combat-based, or at least related to doing or taking damage. Have you considered putting in some abilities that apply to the other two pillars of adventuring: exploration and social interaction? Maybe something that lets you communicate with the plane of fire or an advantage to persuasion/deception/intimidation checks when dealing with fire-based creatures? Or maybe something that lets you see through fires allowing you to make perception checks at great distance that way? Lastly, I think it would be cool if you can think of a fire-specific metamagic and give this subclass access to choose it. That stays on the sorcerer theme and makes this class a specialty as well. Maybe the Forge Cleric can provide a little creative inspiration.

Overall I like it and I think you are making something unique and interesting.

For sparks and embers: I'm concerned about giving it a free spell casting of the highest it can know each long rest including incendiary cloud, delayed blast fireball, fire storm, and investiture of flame. And get rid of the subtle spell effect, just make them have to get the meta magic. 

I like one with fire. 

For fire's wrath: I suggest strengthening it at a higher level to bypass fire immunity as well. Or turn immunity into resistance.

I still like one with fire (improvement).

Fire safety invalidating a metamagic bugs me. Maybe turn immune to resistance.

Give master of flames fire form a fly speed. Enemy fire is possibly too strong, maybe change it to require you to counterspell or dispel magic a fire spell then be able to cast the same spell as part of that action.

Thank you both for the comments, they're very helpful. Based on the feedback provided I have made the following changes:

Sparks and Embers: I removed the once per day free casting, DxJxC highlighted a very overpowered flaw: it was allowing them to cast very high level spells for free every day and that can be a bit much. I preferred to keep the free spell that you can continue to swap for better ones rather than the free casting,

Replaced the Fire Safety feature with Through the Flames based on suggestions made by TexasDevin.

Following advice from both of you I changed Master of Fire and how it works. You can still summon a fire elemental but now you can summon other kinds that are fire-related. You get advantage on social checks for fire plane peeps and your fire damage treats fire immunity as fire resistance. Since the latter two are somewhat mediocre for 18th level I'd reduced the cost of the summoning part to make up for it.

I had to move the One With Fire improvement to 14th level because D&D Beyond has an immensely backward view of homebrew and an improperly designed checking system. (See here). Oh well.

I hope you like the revision.

Since the link above might link to the original version please click here for the new one: Fire Soul