[color:"orange"]Hold the button to charge the spell and release to cast it. You can release the button early to cast the spell at a lower level. Each level exponentially increases the mana cost and effect of the spell.[/color]

I like the idea of connecting casting times, mana, damage and radius to the time a key or button has been pressed. But if there are 10 fireball levels, how many levels are there going to be for non-combat spells? In Divine Divinity, having 5 levels of Wizard's Sight is a waste of skill points compared to having 5 levels of a combat spell. If Larian can't think of a good way to increase a spell's effects, that spell should simply have a lower maximum level or even just one level. Another example is Fade from Sight: Having level 5 is only useful if you stay invisible for some time; if you just want to turn invisible for a second or two during combat, you're better off having level 1. In a system with 10 fireball levels, any invisibility spell should have several levels as well, so it can't be mastered spending just 1 skill point, but that means the effects should somehow become better, apart from a lower mana consumption rate.

Spells like Invisibility or Resist Elements can have multiple levels. They could work like offensive spells: longer casting bring exponentially longer or larger results. Invisibility 2 will last long enough for you escape combat. Invisibility 6 will give you great advantage in a fight. Invisibility 10 will let you wonder through the enemy keep unseen, letting you find the weapon that exploits their weakness in safety.

Similarly for Resist Elements, let them charge it up. Sure, most times you'll charge it up fully before casting, but what if you're ambushed by a wizard who immediately starts casting a Fireball? You only have time to half-charge Resist Elements before he throws the fireball or suffer the full wrath of his flames.

Any defensive spell should have the risk of you being vulnerable while you cast it, but with worthy benefits. The same goes for drinking potions.

As for Wizard's Sight, extra levels are pointless, so it may be worth while to cap it at level 1. Perhaps broaden Wizard's Sight to improve vision in several ways: longer sight in the open and in darkness, and within 25% of your improved sight range you can see through walls, see invisible objects, and see through illusions. Each level improves the duration and range of the vision. Would that be worth additional skill points? Other one-level skills could be given a broader effect to make them worthy of additional skill points.