Balancing Your Feat
I don't think it's terribly well balanced; in most cases, it's actually weaker than a feat:
If you take the first option twice, it's the same as a basic ASI.
If you take either the second or third option, it is strictly weaker
than Skilled (PHB p170), which allows choice of three skill or tool
proficiencies.
Take the first and last option, and it's weaker than Linguist (PHB
p167), which provides +1 Intelligence, three languages, and the ability to
create coded messages.
Feats are supposed to be powerful, and your design is watered down in comparison. Feats also tend to be packaged abilities with a theme, selecting one makes a character different from others with the same class. Were I playing in your game, I wouldn't take anything but the ASI, which I could do anyway.
There are also stacking concerns - normally, the same feat cannot be taken more than once. If this is the only feat available in your game, then it's clearly stackable. It allows a player (over time) to stack a large number of skill proficiencies, potentially having proficiency in everything, which will trivialize skill checks and reduce each character's uniqueness.
The Power of (Variant) Humanity
Variant human is pretty powerful, but so is a normal human. I think you're overlooking the raw strength of solid attributes. Having a bonus to six stats shores up potential weaknesses and/or saves points the player can spend on the character's desired strengths.
New Player Advice
If you're trying to ease new players into the system, you could disallow "Variant Human" completely. That would push off selection of feats until L4 at the earliest (if you allow them at all, they're entirely optional), and allow some time for people to get used to the basics.
Combat-wise, it feels a bit much.
I recently started a druid. My first intent was to find a way to remain effective in combat despite depleted resources ; I found out that the druid hasn't much to offer, in that regard:
- Ranged options (such as sling attacks, or produce flame) lack
range and damage.
- Melee options (such as shillelagh or thorn whip) lack damage in the long run.
- AC is often on the low-end - unless your DM is generous on non-metal armor distribution, or you focus on dexterity.
It is indeed very frustrating, and encourages battlefield control over direct damage. Your homebrew subclass fixes all 3 issues, including strong blaster, healer and tank features: it would have been a no-brainer for me, and that's a warning for balance.
Bladesinger comparison
There are not that many full spellcasters with Extra Attack, and Bladesong looks very close to Heart of Fire. It may seem balanced... until you dig a little more:
\$\begin{array}{|c|c|c|}
\hline
\textbf{Ability} & \textbf{Bladesinger} & \textbf{Circle of flame}\\
\hline
\text{max AC using mundane gear} & 23 & \textbf{25} \\
\text{Requirements:} & \textit{mage armor}\text{ spell} & \text{shield, class level 18} \\
& \text{high dexterity & intelligence} & \text{high dexterity or nonmetal half-plate} \\
\hline
\text{max AC using magical items} & 25 & \textbf{31} \\
\text{Requirements:} & \textit{robe of the archmagi} & \text{+3 shield & armor, class level 18} \\
& \text{high Dexterity & Intelligence} & \text{high Dexterity or nonmetal half-plate} \\
\hline
\text{Hit dice} & \text{d6} & \textbf{d8} \\
\hline
\text{Damage control} & \text{reaction & spell slots-based} & \textbf{healing spells} \\
& \text{Damage reduction} & \textbf{reaction-based healing} \\
\hline
\text{Concentration}& \text{+Intelligence modifier} & \text{better AC = fewer concentration checks} \\
\hline
\end{array}
\$
All in all, the Circle of the Flame druid vastly outperforms its cousin - and can even do so with little ASI investment, should you find a proper armor.
Circle of Land & Circle of Spores comparisons
The Circle of Spores, from Unearthed Arcana, is probably an attempt to offer wannabe-melee druids an alternate to the circle of the moon's combat forms. Let's include it here:
\$\begin{array}{|c|c|c|}
\hline
\textbf{Subclass features} & \textbf{Circle of the Flame} & \textbf{Circle of the Land} & \textbf{Circle of Spores} \\
\hline
\text{level 2} & \textbf{$fire\ bolt$} & \text{one druid cantrip} & \textit{chill touch} \\
& \textbf{1-minute huge melee buff} & \text{Natural Recovery} & \text{10-min small melee buff} \\
& & & \text{reaction-based poison damage} \\
\hline
\text{level 6} & \textbf{Extra Attack} & \text{ignoring difficult terrain} & \text{1-hp zombies} \\
\hline
\text{level 10} & \textbf{+Wis mod to fire, concentration spells} & \text{various immunities} & \text{AoE spores} \\
& \textbf{fire resistance} & & \\
\hline
\text{level 14} & \textbf{turn fire/lightning damage into healing} & \text{beasts & plants hesitate to attack} & \text{various immunities} \\
\hline
\end{array}
\$
It looks like the Circle of the Flame has the best features at each level: best cantrip to allow ranged combat, best melee-buff, extra attack is sick, enhancing concentration-based spells is unique... Even the last feature, that appears situational at first, becomes cheesy once you start to fuel it with your own AoE spells.
Suggestions
So, how do we fix this?
First, you can't aim for balance, and be a strong healer, blaster and tank at the same time. I removed the healing capabilities, as the tank part seemed your main concern, and the theme calls for improved fire damage.
Second, on the AC question: "no metal armor" and bounded accuracy make things difficult.
If your DM rewards you with powerful nonmetal armors, he shouldn't allow big AC temporary bonuses in an homebrew class. The opposite is also true: if he allows big AC temporary bonuses... he should make sure you keep a lowly hide armor for your entire career. Hence the solution offered here - which seems the good way to reliably get good AC, without over-investing in dexterity or "forcing the hand" of your DM.
Here's the result:
Only You
When you choose the Circle of Flame at level 2, fire bolt and green-flame blade become druid cantrips for you. You learn an additional druid cantrip of your choice.
Leaves choices, and includes another fire-themed cantrip. Green-flame blade is mostly aimed at tier 2+ melee: extra attack was too much (especially if you intend to add Polearm Master), but this cantrip offers something comparable to the cleric's Divine Strike.
Heart of the Flame
At 2nd level, you gain the ability to awaken the primal spirits of flame. As a bonus action, you can expend a use of your Wild Shape feature to awaken your inner flame, rather than transforming into a beast form. While this feature is active, you gain the following benefits:
- You gain 3 temporary hit points per level you have in this class
- You shed bright light in a 30-foot radius and dim light for an additional 30 feet.
- You add your Wisdom modifier to concentration checks.
These benefits last for 10 minutes, or until you become unconscious.
Temporary HP is probably the way to go to add survivability to a druid: it matches what has been done for the Circle of Spores and Circle of the Moon. I removed the AC boost: if you insist on keeping it, you'll have to either keep it low (like +2), or remove the ability to use a shield - if you care about bounded accuracy, of course.
Purification through Fire
Starting at level 6, you gain resistance to fire damage. You also learn a purification ritual that allows you to prepare metal shields & armors, so that you can use them.
With your average 14-dex druid, it means up to +3 AC - and an easier access to magic armor in the long run. It is a game changer for most tables, where nonmetal armor is scarce - and can be balanced by making the ritual as simple or as complex as you wish (involving time, gold, crafting skills, a risk to destroy the item?).
Heat of Battle
Starting at level 10, once per turn, whenever one of your spells inflicts fire damage to a creature, you can add your Wisdom modifier to that damage.
Idea is to open that damage buff to more options (e.g fire bolt, green-flame blade) - while keeping it at a reasonable level (only one creature per turn).
Flaming Soul
Starting at level 14, you gain immunity to fire damage. You can also apply your heat of battle on a second damage roll, each turn.
Either affect both targets of green-flame blade, or use it on both flaming sphere and your cantrip of choice.
I'm absolutely not sure that this is well balanced:
- I feel Nature and/or Forge Domain clerics might feel cheated with this, as some of their core subclass features are included here, with few drawbacks
- having both a better AC and temporary HP, while keeping access to healing spells, means that you are tanking pretty hard
- Extra attack was too much - and this may also be. Green-flame blade + Heat of Battle + shillelagh makes a strong combination.
- Bonus to concentration might go out of control if you go for the War Caster / Resilient (Con) route. Replacing it with Con save proficiency or advantage on concentration checks may help avoid those shenanigans.
But I do think it is closer to "balanced", when compared with Land & Spores druids. Obviously, feel free to switch things around (access to metal armor as soon as level 2?) - and take or leave whatever fits best your idea.
Best Answer
Spellcasting
The sorcerer spell list is significantly more damage-focused than the druid list, which tends towards debuffs and utility spells. That doesn't make it inherently unbalanced as such, but it might be of concern. I might suggest using the druid list, and if there are a couple of specific sorcerer cantrips or spells that seem particularly apt for the character, add them to his list, rather than just switching out the entire spell list. It only takes one or two cantrips to give the character a unique flavor, without going to the extreme of rewriting the whole class list.
Wild Shape
Switching Wild Shape into, let's call it "Monster Shape", is a tough question. Honestly, it really does depend on which subclass he takes, because a Moon Druid's expanded CR choices will make all the difference.
A non-moon subclass may find the monster choices very limiting, because there just aren't that many weak monstrosities that don't have a fly speed. I think you'll find that at 2nd level (CR 1/4) he'll have no valid choices, and only a handful at 4th level (CR 1/2). This may, of course, change with future book releases. Once he hits 8th level there are maybe a few interesting Monster Shape forms, but still very limited, and weak for that level to boot. A normal non-moon druid at that level tends to use wildshape for utility effects rather than to get cool combat forms, and some of that utility will be lost just because a Darkmantle or whatever is a lot more notable than a raven, rat, or even bear.
Using Circle of the Moon adds quite a few forms. It's still very limited, but maybe not so bad; the big issue there is as you level up, you'll be adding monsters with bizarre abilities to the list, some of which are fine for their CR in the context of a single fight but may be gamebreaking if a PC can just use them any old time he wants. Most of the really bad stuff is forever out of reach, but at higher levels you will see stuff like Lamias, Gorgons, and Medusae coming into play. And Mimics are in at 6th level, which all by itself might give you pause.
So what can you do? Well, I feel people often try to over-complicate things by rewriting huge swathes of rules text to fit with a weird idea, rather than taking the safer and easier path of reskinning the existing rules with new descriptions to match the flavor of what they want to do, and possibly making one or two little tweaks when the mechanics don't quite work as written.
My suggestion in this case is to use Wild Shape as it's written, but change the flavor by reskinning each of the Beast forms to describe some quasi-human monstrosity instead. A dire wolf becomes a hulking wolf-man. A giant eagle becomes a harpy or even griffon form. A warhorse form is described as a centaur or minotaur, a brown bear is an owlbear or half-troll, a giant bat is the Man-Bat from DC Comics. Whatever. I think that sort of thing could easily give you the flavor of turning into bizarre monsters without getting into the mechanical difficulties of the actual Monstrosity creature type. (You'll still have the roleplay effects of being a gross monster rather than just a regular horse or what-have-you, so this might work best if he goes for Moon druid.)
The only mechanical changes that might come up are whether some monster forms can talk or use their hands, but even then it's less common than it might look. A giant wolf-man that's secretly a direwolf probably can't speak like a human, just growls and howls, and his massive clawed hands could easily be useless for anything but the most basic grasping that a wolf's muzzle could accomplish just as easily. You could have his forms use the "monstrosity" type rather than "beast" -- the only real game effect there is immunity to a tiny list of spells and effects. ("Animal Friendship" and "Dominate Beast" are the only ones that really come to mind, off the top of my head, and those can be largely duplicated by higher level spells -- they're just easier to get to if you're targeting a beast.)