Armorer
I am Iron Man.
I'm going to take one moment here to remind people of something that needs to be addressed before this subclass even starts: There is no reasonable way for a PC to make a suit of Plate armor. None. Like seriously, of all the crafting classes, and subclasses, and rules, that have existed up to this point. This is the one idea that demands it, yet it is still left out in the cold!
Tools of the Trade
Expected, but not unwelcome.
Armorer Spells
A mix of Battlesmith and Artillerist.
Fun fact: Alchemist is still left in the dust without Shield.
Power Armor
Basically a free infusion. The added ability to get prosthetics without having to spend an atunement slot is nice.
Armor Model
Are you a melee unit, or a ranged unit? Fun fact: You can combine these weapons with the Kensei for some bizarre combinations.
I don't think Guardian armor is strong enough to function as a tank. Sure you have regenerating temp HP, but it's back-loaded and requires a bonus action. You could also forgo the weapon to not tank, but that feels like you are just kind of wasting half of your ability.
Infiltration armor is the alternative, for general use.
Also: These weapons use STR and DEX respectively, but you can also sub your INT score for either one. Weather or not you should is a question of your build. As a matter of fact, you can't even use them with infusions till level 9, so it might be better to just use a normal weapon unless you are exceptionally brainy.
Extra attack.
No strings attached this time~
Armor Modifications
Finally, you can infuse your armor and weapons. Also you get two extra infused items for your trouble, as long as you infuse them to your armor.
Perfected Armor
Guardians get a reaction based tractor beam.
Infiltration gets advantage on attacks.
I'll let you judge for yourselves which is cooler.
Thoughts:
This was clearly inspired. I think the infiltration armor's weapon might under perform a bit in the face of a Crossbow. Which is kind of a shame. And this subclass is shut down by Heat Metal, but what are you going to do?
Infusions
More toys to play with.
Armor of Magical Strength
You can use INT in place of STR for saves and Checks.
I don't see this as being really more useful than other armor infusions, even for the Artificer. Especially at level 10 or above.
Armor of Tools
Unlike the previous infusion, this one is totally worth your time.
You integrate a set of tools into your armor (for only 8 hours, what?, infusions should last at least 24). In addition to having the tools at all times, you can also gain your INT mod on tool checks with that tool.
Helm of Awareness
You can't be surprised, and gain a bonus on imitative rolls which is nice if you have room for it.
Mind Sharpener
You can auto-pass concentration saves as a reaction. Dang, that's clutch.
Spell-Refueling Ring
An extra spell slot per day is good.
Thoughts
I could see the want for most of these, and it's great to see that despite the artificer being a new class, there is still stuff in the pipe for it.
Circle of the Stars
“We are made of star-stuff.
Our bodies are made of star-stuff.
There are pieces of star within us all.”
Star Map
It's an alternative focus with some cool aesthetics. The real draw is the extra spells per day you can get out of it.
Starry Form
This is an alternative to wildshape that is vaguely reminiscent of the psionic wizard.
You gain the powers of Bonus healing, Shooting Stars, or Reliable talent that applies to WIS or INT skills and your concentration checks.
Cosmic Omen
It's like bardic inspiration, except it only applies to d20 rolls and you already know what the modifier is.
Full of Stars
You get resistance to weapon attacks. Handy, but is that it all? Like no movement options whatsoever?
Star Flare
It's good teleportation ability, reminds me of the Lurker Warlock.
Thoughts:
I am whelmed by the Star Druid, not underwhelmed, or overwhelmed, just whelmed. It's an aesthetics-first kind of subclass that isn't tied to any lore at all. I suppose I was just expecting a “Druid of the Rose” or a “Druid of the Tree” or maybe even a “Druid of Winter” based on past UA givings. Heck, for a brief moment when I read the name, I thought maybe this was the psionic druid attached to Star Spawn or whatever kind of Star that the Mindflayers were running from. This could have been the missing link in lore, but no, its just a big ball of flaming gas.
Fey Wanderer
Taking a trip on the wild side.
Fey Wanderer Magic
More magic is more good. And this is a solid list.
Cunning Will
Advantage on saves could come in handy, and the extra skills especially so.
Dreadful Strikes
And now for the first oddity: You can deal 1d6 extra damage per attack, but only once per creature per turn. Which encourages you do spread out your attacks, but are you really going to do that? At least it gets around a bit of Bonus Action Scarcity by not conflicting with two weapon fighting, but then again the damage restriction makes it so two weapon fighting isn't that great anyway.
Blessings of the Courts
Ranger Smites, but why are they 3d6 regardless of spell slot used? The WIS bonus to CHA checks is also nice.
Beguiling Twist
Kind of a nice way to throw salt in the wound of a creature that tried to charm you. Unfortunately your party isn't any harder to charm than normal.
Misty Presence
Selective invisibility, against one target, who gets a saving throw against it. A 24 hour duration is interesting, but this is by no means capstone worthy, when the normal Invisibility spell is arguably better.
Thoughts:
This subclass has some interesting ideas and themes, but the experimental mechanics really snag up the class.