So a Codex is basically a series of encyclopedias collectively called a Codex that is available through an in-game menu that offers background lore and info about a world. Usually there's a bestiary, location synopsis of visitable areas in the game, item lists, and if there's any crafting in the game, a recipe list.
Progressive Unlockable Content simply means that you don't start with a completed codex but you fill it out over time by actions you do in-game. For example you don't get a bestiary entry on an enemy until after beating said enemy, you don't get recipe info on something until you have learned the recipe, etc.
Now I think most people would agree that a Codex is not a bad feature as it adds an unobtrusive way of expanding game lore and provides a quick reference for players to use in-game.
I'm curious more about what kind of information players would most be interested in having access to in a codex as well as how you feel the information should be unlocked. Should entries be robust and give detailed info on stats, drops, shop info, etc. or just basic synopsis?
Personally I am a bit partial to the Final Fantasy XII system where you got basic bestiary info for killing said monster, but a number next to the entry showed how many of said monster you had to kill to unlock additional info about the world's locations or history. It added an interesting element and for lore buffs and completionists it gave them a reason to kill multiples of every enemy type. My only grievance with the system is that the history and location info was not segregated from the bestiary but instead tacked on as secondary pages to bestiary info.
What's your opinion on this feature and it's best implementation?