Feruchemist

Feruchemy lets you store attributes, such as strength, weight, memories, health, and more inside various types of metal objects. "Storing" involves decreasing that ability for a while, then increasing it by the same amount sometime later. A Feruchemist can usually only access their own power in metalminds, not someone else's. Two people can store in the same one, but they can't take from each other. There are 16 metals that can be used as metalminds, with 16 corresponding abilities.

Give and Take

Different types of metal allow for different abilities to be stored. Luckily for you, a full Feruchemist has access to all of them. When you touch a metal object of one of the following types, you can store the corresponding attribute inside it, weakening you in some way. When you're touching the same object during your turn later, you can take back the stored power, increasing it by the same amount as before, or you can continue storing. I would recommend keeping track of how much power is in the metalmind by using a points system, adding the corresponding amount of points to yourself or the metalmind at the end of each turn. Any points added to yourself disappear at the beginning of your next turn for most metals, so act wisely.

Due to various magical influences, you lose 1 point out of all of your metalminds each hour. In addition, you cannot use more than 2 metalminds at a time without causing significant strain on your body. Before removing the points at the beginning of your turn, take damage equivalent to 25% of your max HP for every metalmind you're using minus 2. 

Iron: When you touch any iron object, you can store weight inside of it, making yourself lighter. 1 point = 25% of your weight. (no you cannot make yourself lighter than 0% of your weight) Storing 25% multiplies your jump distance by 1.5, 50% doubles it, 75% quadruples it (*4), and 100% multiplies it by 16 because your clothes still have weight. When you take, your movement speed is reduced by 5 feet for every point. Increasing your weight by 100% halves your jump distance, and so on, do the math yourself if you decide to jump with more than that.

Steel: Storing inside steel would diminish your physical speed, while taking would increase it. 1 point = 5 feet of movement. You can also use 1 point to take half damage on an attack outside of your turn.

Tin: Senses are stored inside of tin: sight, smell, touch, taste, and hearing, as well as the ability to feel pain or pleasure. You can only store one type of sense in a tinmind at a time, requiring another one for another sense. 1 point = +/-1 on skill or ability checks relating to that sense.

Pewter: Pewter allows physical strength to be stored inside. 2 points = +/-1 on any STR or STR-based skill checks.

Copper: When you touch any object made of copper, you can store memories inside. While storing, everything you do, see, or hear will be recorded in the metalmind. When you stop storing, you will forget all of that from after you started storing. You can also put your memory of any event you can currently remember into a coppermind, immediately forgetting it. You can recall the stored memory with perfect clarity when you take it out of the coppermind, although it will fade like a normal memory if you don't put it back again. There are no points, just write stuff down if you use this. The memories do not drain from the metalmind, unlike the others.

Bronze: Storing inside bronze would diminish your wakefulness. With enough wakefulness drawn from the bronzemind, a Feruchemist could even overcome sleep-inducing spells (DM can decide how many points are necessary). When you would fall asleep naturally, it takes 1 point each 5 minutes to prevent falling asleep, rising to 2 after half an hour, 3 after an hour, and so on. If you store 4 points at once, you will fall asleep.

Zinc: Mental speed is stored inside zinc. This essentially boils down to your ability to reason and how much thinking you can do in a certain amount of time. 1 point = +/-1 on INT- and WIS-based checks, as well as time outside of the game to think about your turn, although you can probably get a ton of that anyway. Warning: don't be annoying with this.

Brass: Brass allows the feeling of warmth to be stored inside. 1 point = I dunno, +/-2 degrees? It doesn't really matter how much you assign to each point here.

Gold: When you touch any object made of gold you can store your health inside. This allows mostly for rapid healing. Storing diminishes your CON modifier by 1 and your max HP by 10 for every point, while taking adds 1 to your CON modifier and heals you by 10 for every point. The heal comes when you start taking or at the end of your turn. You do not reduce your hit points when you stop taking.

Electrum: Storing inside electrum would diminish your determination, making you depressed and lazy, while taking would increase it. Taking 1 point would reduce all damage taken by 2 and increase damage dealt by 1, and vice versa.

Cadmium: Breath is stored inside cadmium. The Feruchemist has to hyperventilate in order to get enough oxygen while storing. 1 point = 30 seconds without needing to breathe.

Bendalloy: Bendalloy allows energy to be stored inside. "Energy" means the energy you get from food and hydration you get from liquid. Food and water cannot be stored in the same metalmind. 2 points is like a snack, 8-10 points is like a meal. 1 point = 1 cup of water. You do not gain or lose energy after you stop storing or taking.

Aluminum: When you touch any object made of aluminum, you can store a small piece of your identity inside. This allows you to create "unkeyed" metalminds that contain power can be accessed by anybody with the corresponding Feruchemical ability. Once claimed by someone, the unkeyed metalmind will become theirs. To do this, you must store whichever power while also storing identity. Also, treat any magic items you're attuned to as if you weren't while storing.

Duralumin: Storing inside duralumin diminishes your connection to other people, while taking would increase it. Your relationships with other people weaken and some people might even forget about you for a bit. You can draw upon this power later to speak foreign languages and make fast friends with strangers. Storing 1 point means all acquaintances become strangers, 4 points makes all new or okay friend strangers for a bit, DM can decide how much NPC's care about you and assign point values.

Chromium: Fortune is stored inside chromium. You become lucky or unlucky when you store or draw in from your metalmind. 1 point = 1 reroll of any dice that affect you, you pick which result to use. Storing 1 point means you reroll each dice 1 time, taking the worst one.

Nicrosil: Nicrosil allows Investiture to be stored inside. This means you store your magical energy and connection to the world in the nicromind. You can store Feruchemic and Allomantic abilities in the nicromind, although this is more or less useless for someone who only has Feruchemy. 1 point = 1 spell slot as well. You need a separate nicromind for metallic arts than spell slots. You cannot use any other Feruchemical or Allomantic abilities while storing inside nicrosil.

Reflexive Intake

As a reaction, you can draw in up to (fighter level - 7) points from one type of metalmind. The normal amount of points will be lost at the start of your turn. You can do this once per short / long rest, twice at level 12, and thrice at level 16.

Phenomenal Metals

As a level 15 fighter, you can have discovered the ability to use the following 4 metals for metalminds:

Mithral: When you touch any object made of mithral, you can store reflectivity. While storing, your body, clothes, and whatever you're carrying become more transparent, and drawing the power makes you reflect more light, basically becoming a large oddly-shaped mirror when taking enough. -4 points = invisible, +16 points = you're a mirror now. +1 to stealth checks for every point you store, and vice versa.

Valythnium (a metal that I made up because I can't find an alloy of mithral): While storing inside valythnium, your size is diminished. The amount you store directly relates to how much you and anything on your person shrink, and the same applies for growth. You can basically be Ant Man. 1 point from Tiny to Small, 1 point from Small to Medium, 2 points from Medium to Large, 4 from Large to Huge, 8 from Huge to Gargantuan, 16 more points for every foot in diameter you grow. Same goes for shrinking (1 point from Medium to Small, etc.)

Adamant: Skill is stored in adamant. You lose skill with every weapon, language, or instrument while storing, and tapping makes you proficient. Note that each of the aforementioned categories must be stored in a separate adamind (weapons in one, languages in another, etc.). 1 point = proficiency / disadvantage with unknown skills, doubled proficiency bonus / no bonus with proficient skills, etc. Next would be triple proficiency bonus, then quadruple, and so on.

Adamantine: Adamantine lets you store resistance. While you store power in the adantimind, you have vulnerability to all damage sources except for slashing, piercing, and bludgeoning, assuming you have no resistances or immunities to those already. When you take power, you gain resistances to the aforementioned damage types. 1 point = 1/2 or 2x damage, 2 = 1/4 or 4x damage, so on. When you reach 1/64 damage (6 points for a damage type without natural resistance or vulnerability) you gain immunity.

Metal Mastery

You know how to most efficiently use your abilities. For every 4 points stored in a metalmind, add 1 free.

Control

You can use your reaction to stop storing or taking power from ONE type of metalmind. The points lost or gained by you or the metalmind are divided by two, and resolved immediately. You cannot do this after 2/3 of the initiative count between your turns has passed. You can do this once between turns until level 16, when you can do it a second time.