I've just thought of an idea for the spell interface that could make sense.

Titan Quest has a similar system to Original Sin, in that it has a general Specialist skill that you're required to increase to unlock higher tier skills, similarly to how you need points in Way of the Warrior to be able to use higher level warrior skills. It also doesn't use skill trees (Squares are active skills, Circles are passives, circles attached to Squares modify the base active skill, but are not active skills themselves).

It represents that by having a bar along the side that shows your current mastery level, and a bunch of skills at various levels.

Laying out Original Sin's skill interface that way would help people plan and see what skills are available and when.

That's a better approach than locking all the other tabs for spells that you have no points for and hiding what you haven't learned. If there are secret skills which you need to unlock somehow, they could show up as icons with a (optional) name and description that only says "Unlocks when you discover a certain thing".

If there's a secret tab, an entire branch of secret magic, that is where a locked tab could be used.