Deep in the darkness, chests of treasure lie, left long ago by persons unknown, and guarded by terrible beast. Yet, why seek such dangerous bounty? What do these ancient hoards hold, that you, a master of the crafting arts, cannot make for yourself? Perhaps they hold more than you know...
With the random loot available in Vanilla, there isn't really any reason to go and seek out dungeons. Most of what they hold is junk, or only middle-tier at best. The few good things in them, you don't need much of. Thus, the purpose of "Treasures!" would be to give people a little more incentive to go looking for dungeons. To accomplish this, it would introduce some new and useful items that can only be found through exploration.
The basis for this idea is actually from an old suggestion thread I made, Waybread. The suggestion was for a special food item, only available in chests, which could stave off hunger for far longer than ordinary food, thanks to max hunger and saturation values, and a unique status effect that would temporarily prevent all hunger loss. I would actually like that to be the first treasure on my list...but what else?
Starmetal: This is a dark jade metal which, for the purpose of lore, we'll say fell from the sky as meteors in ages past. Rare and highly prized, it will show up in chests in two forms. The first and most common is piles of nuggets...I'd say a maximum of three, maybe four per pile. The other is as crafted tools and armour. Starmetal equipment has only the harvest level and durability of iron, but has the damage and defence levels of diamond, and the mining speed and enchantability of gold. About 80% of the time, it should come pre-packaged with low to mid level enchantments. Two particularly rare pieces of Starmetal equipment would be the Starmetal Hoe and Starmetal Shears, each of which always bears a unique enchantment when found as treasure. The hoe has "Growth I" which allows it to be right-clicked on plants to apply a bonemeal effect in exchange for durability, and the shears have "Providence I" which gives a chance of dropping more wool than usual when shearing sheep. Neither enchantment may be gotten any other way.
Lost Magic: These are special enchantments, which can't be obtained by using the Enchanting Table, even with things that can be given them at an anvil (like how Axes can be given Sharpness, but not gain it directly). They will appear rarely on items, with Starmetal items slightly more likely to have them than normal ones. Otherwise, they will be on books, distinguished from Vanilla enchantments by a book icon that is charcoal coloured with a purple ribbon. However, there aren't many of these.
Hero's Might (Melee Weapon, Max level 7): Weapon deals increased damage to bosses. Cannot be applied to a weapon with Sharpness, Smite, or Bane of Arthropods.
Soultap (Any Weapon, Max level 3): Mobs drop more experience orbs than usual. Weapon has a chance of losing double durability on use.
Tempest (Bow, Max level 1): Projectiles trail water particles, and do greatly increased damage to End and Nether mobs. Cannot be applied to a weapon with Flame.
Lunatic (Any Tool or Weapon, Max level 1): Increased damage and mining speed at night, depending on the moon.
Shroud (Armor, Max level 3): Reduces the ability of mobs to detect you. Cannot be applied to armour with Protection (but Fire, Blast, and Projectile Protection is fine.
Idols: These are small statues that can be placed in the world. Normally, they do nothing. However, if placed on a block of Lapis or Gold, or on a small pyramid (two layers) of Obsidian, it produces effects in a radius around it. The radius is smallest for Lapis, and biggest for Obsidian. Most have only minor effects, like occasionally not losing hunger points, or crops growing slightly faster. Some, however, have stronger effects that come at a cost, like a passive bonus to drops which stacks with Looting and Fortune, but mobs in its area of effect hit harder and take less damage. There is no way to know what effects an Idol will have unless you set it up.
Treasure Compass: (Not sure how practical this one is...) When right clicked, it will lock on to the nearest chest in its range...probably within the same chunk. So long as right-click is held, it will continue pointing to that chest. However, it will also lose durability the entire time it is in use. Should not have too much durability, and might typically be found already damaged.
Lost Potions: This would just be potions for any status effects that are in the system, but don't have a craftable potion attached to them. Pretty straightforward. More information here, from the Minecraft Wiki.
Ancient Lore: Scrolls which can be read like a written book. Could contain stories, maybe taken from the less modern tales on SOG Wiki and Creepypasta Wiki, or from folklore from around the world. Some would also contain stories providing mostly accurate information on things from the mod that players might need to know, like how to use Idols, or where Starmetal came from.
Aqua Vitae: These blue bottles resemble a Bottle o' Enchanting in form and function. When right clicked, they are thrown, and any players or mobs in the splash gain empowered and extended Absorption. In addition, they apply a Bonemeal effect to every block in their splash radius, and puts animals into "love mode".
Tapestries: These would be something like a cross between a painting and a music disk. Individually numbered, each would have a specific painting, and list the title and artist of that image, as well as the wall space it covers. For example, "Van Gogh - Starry Night (2x3)". Purely decorative.
Ancient Materials: These piles of gears and other nonspecific junk could be used to repair any item to full durability in an anvil. Using them might cost less than normal as well, or even reduce the cost of future repairs (assuming that's possible).
Protective Charms: These would come in several types. Some would reduce damage from specific sources (such as undead, drowning, or falling), while others would prevent specific status effects. All would work the same way. Right click it in your hotbar to activate it, and so long as you carry the activated charm with you, whenever its condition is met, it will lose durability, applying its effect. Like the Treasure Compass idea, they would only have small amounts of durability, and usually be damaged already when found. The rarest of all would be a death protection charm, which would prevent all damage and heal the bearer to full when they would otherwise die, but breaks after a single activation.
Other ideas in a similar vein could be added, but I thought that this alone would give people a great deal more variety in the things that they find.
*shudder* I cannot read the words "Aqua Vitae" without having flashbacks to Amnesia.
That aside, I like the idea of having the "lost potions" added to the game proper before they're officially craftable.
And I love the idea of adding a decent supply of painting-like objects (tapestries is a fine way to look at it) that let resource packs make up dozens of new decorative bits. For a long time I've wondered why paintings get treated as a spliced texture instead of being made into individual textures, where you could easily swap out old paintings for new without having to cut and paste bits of a picture and line them up pixel-perfect. Having additional painting-like resources could allow for dozens of new bits, from actual paintings to curtains and door-hangs to miscellaneous decorative "shelves" and wall-hangs like some resource packs already make the paintings into.
Other than that, your items don't sound bad, just not my taste. I'm currently looking for some dungeon-generator mods that'll add new places to explore without adding either new monsters or overflowing pages of new loot, because I have over 60 mods now and having loot to choose from is so not my problem. Plus, one of the neatest dungeon-creator mods I've run across (ChocolateQuest) not only throws some errors at me but appears to be the one that throws up a gigantic page of loot whose graphical style looks absolutely nothing like vanilla, and it's bugging me enough that I'd prefer not to add it back to the list :\
Note: I'm actually not sure that CQ is the one giving the giant loot page. It just vanished during one of the mod-list parings that cut out CQ and I was too tired to double-check it.
My YouTube channel is currently on hiatus, but I hope to get back to it at some point. Content is fairly random, but can be enjoyable, and is mostly game footage (mostly random Minecraft clips) from my nephews and me. Most popular MC vid so far is the one Vechs laughed at on Twitter!
It's been a while, and I'd like to open this request back up again. This is still something I'm looking for, and would love to see done. With any luck, this could go into a modpack I've been sitting on unfinished for a long time. If anyone is interested, please, let me know!