[question about using Feruchemy in Dungeons and Dragons]
Why Feruchemy "works" in book terms is because it's about intrinsic trade off. We see the character pay something, so we accept when later on, they're able to do something dramatic. Narratively, their boost is "earned" in much the same way that a character "earns" their ending winning a duel by showing us through the story that they've been practicing with the sword.
You need to "earn" your boosts. If I were a GM, I'd suggest that you can store attributes during one day of gameplay, to use it during another day of gameplay. -2STR for one day, +2STR for the next day. I'd say no more than -/+2 at first--with feats or Feruchemist prestige class levels allowing you to do 4 or even 6. Storing senses could be covered with WIS, and health with CON.
Alternately, if you want to get into the specifics, you could try something where when you land a hit, you can use a smaller damage die (a d4 instead of d6) to "store up" strength. Then later, when you need it, you can trade in one of those stored moments (which would be capped with a maximum number that could be stored at once, to be raised by requiring you to find special metals) to raise a damage die during a climactic battle--maybe making your d6 into a d10. You could do the same thing with spot checks (take a penalty for specific rolls to be able to add to the later on.) HP could be done the same way--drop your HP for a battle to "store" then raise them for another battle.
This is more of a tweak to the way the books use the magic, but the idea is to make certain your cost is still a cost. You get ahead by choosing the times to - or +, making it fun--but you are always paying a price.
So the first question I'd ask myself is do I want this to be a time period thing or a specific instances thing--which would be more fun to play? Then ask is this about attributes or specific skills/hit points, etc? Define some rules, define how you get better, and then have fun within the system.
Personally, I'd avoid the will save as a cost to drawing out the attribute or ability. Perhaps make it require concentration checks if you want to make it tougher--but requiring a will save to magically gain strength doesn't feel very "feruchemist" to me and downplays the real fun you could have with the character. Role playing a day spent with very low spot checks, or a terrible constitution, could be really fun.
I'd also figure out if you can do some kind of "super move" with the abilities by storing up a whole lot. (Like ten units, however you decide upon them.)
My take on the attributes: Iron: To be used in a role playing way, making yourself lighter or heavier, with no battle implications. Steel: Increase/decrease movement speed in a fight. OR under the effects of a "slow" spell for a day, vs under the effects of a "haste" spell. Super move: Very limited time stop. Tin: Spot Checks or WIS. Pewter: STR checks, damage die, or +/- damage to each hit. Zinc: Bonus to hit (for thinking through the situation) or bonus to initiative. (With corresponding negatives.) Brass: Specific fortitude checks.Copper: Mostly role playing. Memorize a book, or an entire library, if given time. Blank things from your mind to prevent mind reading. Bronze: Mostly role playing, with (perhaps) being able to "rest" immediately and get back any abilities that come with it. (Haven't played 5e--these were big in 4e, but don't know if they kept them.)
These metals are going to be rare.
Cadmium: Not having to breathe for a time could have all kinds of applications, though I'd love to hear you role play hyperventilating all day for one session. Bendalloy: Not eating and storing calories. Great for role playing.Gold: CON bonus, hit points, or something like that. Sudden healing is great for gaming. Electrum: General bonus to all skills. Chromium: Bonus or minus to any roll.
The rest aren't even understood in-world, so I'd stop there. If you go all in on this, I'd say you need some kind of class built around it--perhaps a rogue or monk base, replacing their bonuses with feruchemical abilities that you gain over time.