It would also fit smoothly with the upcoming Battle Rework and could find some space to slide it in there, as we won’t be needing Silver to level up attributes no more but points instead.
Hidden classes can instead become a benchmark " Amplifying " boosts for existing or new skills and attributes. What would be great for it is that, if you could take out the offerings to level up the hidden skills or attributes any time or at any new Rank, to mix-match skills. That way the player doesn’t feel forced to stick to just one Circle and opens up a vast variety of outcomes.
Not only would it make your character more suitable for situations but it would make PvP even more interesting, not knowing what Hidden attributes they hold and actually devising a strategy to take them down.
You should also be able to switch Hidden skills or attributes on the fly, inside the skill window, instead of at the Master NPC. It would save time for travel or even save your skin depending on the situation. For PvP though, every new Round you have let’s say, 30 seconds up to a minute to prepare your Hidden attributes to suit your gameplay, then switch it up every Round to counter-attack. I think this will be a better use for the Hidden Classes.