One of the major advantages of GURPS is creating your character the way you want, since the rules let you create anything. And the spell-casting system is arguably too complex or boring.
Putting together those two ways of thinking, there is a sub-system that I inherited as GM where the players can cast spells without the casting time. Plus they don't have to learn spell by spell to do something like tossing a fireball; there is a "Fire Manipulation" generic spell where the PC that acquires it can freely manipulate fire (or any other element - according to the skill) having the cost in mana scale up to how much he wants to manipulate.
That's obviously too OP, but I managed to balance it with the encounters in the adventures so far. The problem is that I'm reviewing the rules of this sub-system we are using, and I'm having trouble calculating how much it would cost in character points for such a spell.
Such a spell would combine multiple spells of the basic system into one, plus remove the combined casting times. To exemplify: One could cast a fireball at cost of 1 mana, or cast a wall of fire having the cost in mana depending on how many hexagons it would occupy.
How many character points would such a skill cost?
This ruleset could apply to 3rd or 4th editions.