Dungeon Master Guide, page 165
Dragon Scale Mail
Armor (scale mail), very rare (requires attunement)
While wearing this armor, you gain a +1 bonus to AC, you have advantage on saving throws against the
Frightful Presence and breath weapons of dragons,
and you have resistance to one damage type that is
determined by the kind of dragon that provided the
scales (see the table).
Additionally, you can focus your senses as an action
to magically discern the distance and direction to the
closest dragon within 30 miles of you that is of the same
type as the armor. This special action can't be used
gain until the next dawn.
The armor has also base features of a scale armor, which means:
- it will grant AC of 14 + 1(magic) + Dex modifier (max 2)
- disadvantage of stealth
If it is not enough, you could make this item legendary, by increasing the armor bonus by 1. Then it would grant 16 + dex mod (max 2).
If your character relies on stealth and high dexterity (16+) you could get feat Medium Armored Master to negate armor's disadvantage and increase max dex cap to 3.
Anyway it appears that you have missed the most important Rule 0.
The D&D rules help you and the other players have
a good time, but the rules aren't in charge. You're the
DM, and you are in charge of the game. That said, your
goal isn't to slaughter the adventurers but to create a
campaign world that revolves around their actions and
decisions, and to keep your players coming back for
more! If you're lucky, the events of your campaign will
echo in the memories of your players long after the final
game session is concluded.
Dungeon Master's Guide, page 4
Your players may dislike it but, as Game Master you can create anything that you would like to. Although you shouldn't ruin the fun for everybody, as it would be simply immature.
Druids have problems with metal equipment? Why won't you make a full plate armor made from draconic bones, skin of Tarrasque, scales of Bahamut/Tiamat or pure diamonds. Anything is possible, use your imagination! That is what role playing is about.
You should try to find out the consequences yourself, however if you find it difficult, there are possible consequences, which I often use:
- The weight is increased or decreased, maybe even multiple times in comparision to the original.
- Change of the value.
- Unique materials are needed to maintain the armor's performance. Blood, grease, saliva, extract from unique species.
- (for magic armor)The items work only, when all pieces of the set are gathered.
The more positive aspects may be based on already existing ones or the source of the armor.
Examples:
- Granite breastplate weights 3 times than normally (60 pounds), costs 300 gp and has a disadvantage on stealth tests.
- Shell full plate armor costs 2000 gp, and weights 50 pounds and must be regularly treated with Plesiosaurus' mucus.
- Diamond full plate armor is priceless, gives immunity to critical hits and resistance to physical, non-magic damage.
- Hide made from Sahuagins gives advantage for underwater stealth tests.
It's fine, but its awkwardness gets in the way.
Total, the strength of this weapon is effectively:
- No proficiency to hit, but instead get a "+2 to hit", which makes its accuracy worse when your proficiency bonus is +3, unless you're not proficient in martial weapons (in which case you wouldn't get proficiency to hit anyway, and thus makes this a +2 weapon).
- +2 damage
- Can deal 1d4 lightning damage, once per enemy (and once to yourself). Assuming average damage (2.5) and it zaps 40% of the time (generous) that's 1 bonus damage.
- Otherwise a fairly normal Glaive (in terms of power).
So, is it balanced? I guess? I wouldn't use it once my proficiency is +4, as my +2 to hit difference is too valuable compared to the +2 damage this spear provides.
I would never throw the thing for 1d4 damage that's nullified on a save. If I happened to roll a crit fail, depending on how you'd rule it, it could just fly off into the horizon, never to be seen again.
So in the end, it's just a fancy/weird glaive, but I'd rather just use a normal glaive for consistency, to be honest.
The main issue isn't the balance, though. It's the rule keeping.
At this point, with this weapon, you have to remind yourself that:
- You can't apply your Proficiency bonus
- You have to remember to roll 1d4-1 to hit every time you attack with it.
- It has a +2 to damage, separate from its to-hit "bonus"
- It deals 1d4 lightning damage on contact, with a wisdom 15 save, but only the first time it touches a creature.
- I have to keep track of which creatures I've already struck specifically with this weapon (would be confusing if an ally picked it up and started using it after I threw it)
- I have to remember that it has a throwing range, but it only deals damage on a crit, and only deals normal damage on that crit+1d4. And if I happen to miss, it just keeps going.
And...for what? Roughly +3 damage and roughly -1 to hit.
I'm all for complicated things that are worth the work and revolve around individual decisions to make (like the Battlemaster), but this overcomplicates a number of things for some extremely underwhelming benefits. That may be intentional, but your players will use it once or twice, and then get frustrated when they find this information out first-hand that it's not good as a weapon. You could simplify it while still keeping it weird, or you could turn some of these complications into tactical uses, but as of right now, it's mostly just a fancy paperweight.
Best Answer
There are two sides to this issue that I can see. Firstly, the most obvious one:
It devalues classes that get the Extra Attack class feature
Fighters, Barbarians, Paladins, Monks and Rangers are less special in that regard; although you are allowing the second attack only if the first attack misses, this is still something these classes could do anyway (if they missed their first attack). Increasing the likelihood that any other class can recover from a bad roll will devalue these classes somewhat.
Regarding damage comparisons between these aforementioned classes and others, Rogues keep up with the damage output of these aforementioned classes via Sneak Attack, and casters keep up with cantrips (see this related question/answer).
The classes with Extra Attack cannot keep up with the Rogue as well if the Rogue has a new way to recover if it misses, potentially dealing all it's sneak damage even after rolling badly on the "first" attack, whereas the Extra Attack classes can, at best, deal half their usual output of damage with the same rolls (i.e. a miss, then a hit). The same is true of a caster (that can also swing metal around, such as a Bard or Warlock) if they can opt to try their luck with a weapon they get two attempts with, rather than an all-or-nothing cantrip attack roll.
Comparison to magic items
The other issue I see is that this is comparable to the Scimitar of Speed, which is a very rare weapon. It is also a +2 weapon, and since +2 weapons are rare, you could argue that a Scimitar of Speed without the +2 might then be Uncommon (or perhaps rare, although given that yours only "activates" on a miss, probably more likely Uncommon), but regardless, having a certain kind of metal in your game that allows the property of a magic item is also quite unbalancing, since it's something where something comparable currently only exists as a magic item.
Regarding balance, though, this depends on how rare this kind of metal will be; if it's going to be equivalent to an Uncommon magic item, then it's rarity might be comparable to that of magic items, which reduces the balance concerns (compared to if it were as available as mundane weapons).