I like the idea of having some unique things hidden in different parts of the world, I have some plans for the 4th rarity level of items that would make them more unique, so maybe spawning a random one in hidden, random places could give extra incentive to explore.
Not sure about the keys, I like the idea of a completely open world and not having to go to any place or track down a specific person to progress, maybe as some bonus stuff.
As for the possibility to implement, I have a custom engine and performant language at my disposal, so given enough time almost anything is possible. :) The significant restrictions will be time and priorities, but I plan to stick to Soulash for at least next few years so I'll have enough time to implement great ideas. Feel free to share anything you can think of.