Just a quick idea, still needing a few details here and there, anyway what do you guys think?
Silver Chest & Gold Chests
These chests contain rare items and or upgrades to Link's current inventory. They can be found in parts of the overworld, and or locked away in small and yet secret parts of houses, with or without residents living in the house.
Opening them
The only way to open them is by finding extremely rare silver/gold keys to open a chest of your liking. The keys are given to the player via side-quest and or found in game. They are also rarely given to players by completing a Mini-Games and or auction(if it is in KoT).
Obtaining a Gold/Silver Key
When obtained, it's true useage is not explained in text, so the player needs to figure out what the key is capable of unlocking.
*Side_Quests - Completing a side quest at times, NPC rewards the player with a gold and or silver key.
*Mini Games - It will be a prize which is given and or chosen by/to the player.
*Found in small insignificant chests that are well hidden scatter in the world of KoT.
*Hidden well within the overworld and or dungeon/cave/house areas (Kinda like in the Fable Series).
The keys can be found in hard to reach areas within the overworld and or inside a dungeon. This gives the player a "come back later feel" so that the dungeons are not always a finish and forget about it kind of thing.
Content
Like I said, they will contain items such as high amount of rupees, and or special extended upgrades to the player's current inventory. For example, if there is no upgrade for deku nuts, and will likely find one within a gold/silver chest.
Origin of this Idea
*In the Fable series on Xbox, The silver Chests can only be open by using a silver key to unlock them.
*LTTP Big chest idea by using a big key to open it. The key severs as a boss key as well.
*They are similar to Chrono Trigger's black chest(s).
*Also other RPG games that show locked chests and they can only be unlock via special key.