It could be interesting. Take it even a step further and have all slots be unlocked. The player would basically build the character's abilities by purchasing them.

You could unlock armor slots individually and upgrade them to bigger and better things. For example, you could unlock the hand slot, but they could only wear light armor on their hands which would be something like leather gloves. Then you could upgrade it to a medium type, which could be something like chain gloves. Then upgrade it to heavy so they could wear full plated gauntlets.

You could also do about the same thing with character skills like magic and what not.

A strong magic user would be good at magic not because 'the developer says so' but because they spent their valuable xp on increasing their magic abilities at the cost of not increasing their weapon usage abilities or armor wearing abilities. The strengths and weaknesses that are normally built in would come naturally from how the player has decided to evolve their character over the course of the game.

Alternatively, if you go the route of armor and weapons infer attribute bonuses and what not like in WoW for example, you could have all slots open, but they are for only clothing and very basic weapon types, like a staff/walking stick.

Another way of thinking about gear is to create sets of gear aimed at specific environments. Instead of replacing/equipping one item at a time, you would choose a "set" or "costume". Then there would be some editor where you could create and name these costumes. Of course, players would mostly want to wear the most optimal equipment at all times, so this might not be a good idea for your particular game.

I think you can make it work. Like you said, its all in the execution. The first (good) step you've made is realize you can't start the game with all of these slots opened. This would be overwhelming.

On the other hand, I kinda like how games such as La Mulana have managed their inventory, somehow forcing you to make choices (very similar to Zelda a Link to the Past).

Also, do you plan your equipement pieces to be passive or play an active role in the game (additionnal input, etc).

You need to gauge that level of complexity, and gauge it in a way where the player actually has to make choices (not just unlock a new slot and equip whatever "objectively best" item they can).

My suggestion would be to play it like a "robot" type of game, where you need to return to a specific area (manufacture) to change your equipment. Since you can't change pieces on the fly, your choices actually matter. You could easily theme this idea differently, but I think the result is there. That way, some players will choose to play it risky and use very specialized items that suit their need, whereas other players will settle for more general-purpose items to insure they don't come across unexpected obstacles they cannot overcome.

What type of game would you implement this in?

maybe think about *why* you usually got annoyed at games-with-unlockable-item-slots, maybe because you couldnt play/explore all the option ? this would be largely negated in a game where you have a small army :)