A basic spell sets up the basic powers or limits of a spell and we will build upon this as we calculate the 'cost' of a spell. A Magic-User would be required to make a successful Saving Throw (ST) to successfully cast their spell as they desired. A spell configured at the basic level requires a ST roll equal to the current ST for the caster.
Basic spell Calculation:
Die effect (Power?): d6 (or modifier to target's Saving Throw)
Range: 10 yards/meter
Duration: 1 round (or instantaneous)
Area of effect: 1 target (equal to one person's mass, could also use Hit Dice)
Casters would use their ST as a target to roll or beat, plus or minus an applicable ability bonus. If the roll is a failure the spell is not cast, GMs can have them simply fizzle out or come up with some crazy DCC style spell failure table if they like a little gonzo in their game.
Example: Bryan wants to cast a fireball. His St is 15 and he has a +1 bonus to his roll from his Intelligence. He needs to roll a 14, plus his +1 modifier, to cast his fireball spell successfully. If he is successful, his fireball goes 10 yards, does d6 damage to one target, and lasts just this one round.But how to cast more powerful spells? We use what I am called "steps". Each attribute above for the spell is a baseline that we can raise in steps to increase the power of a spell, but this increases the difficulty. Raise the Die effect of a spell to 2d6? That costs a -1 to your roll. Increase the range to 20 yards? That costs a -1 to your roll. Want the effect to last longer, say 2 rounds? That is a -1 to your roll. For simplicity, keep the ST the same and apply all modifiers to the die roll.
Each step effectively doubles the previous level. In other words, Die effect ladder is d6, 2d6, 4d6, 8d6. Range ladder is 10 yards, 20 yards, 40 yards, 80 yards. Duration ladder is 1 round, 2 rounds, 4 rounds, 8 rounds. Etc, etc. I am still debating on this as this doubling might cause magic to become extremely powerful...it requires a good deal of play testing. I may decide to use a more simple step such as d6, 2d6, 3d6, 4d6, etc. My main problem I have with this concerns the duration, many spells last longer than 1 round but jumping from 1 round to 1 turn is a massive jump, combine that with a d6 spell and you have a seriously major kickass spell. I will need to think on this more.
Example: Bryan is desperate and needs his fireball to really pack a punch. He Decides to raise the damage to 2d6. Now he will need to roll a 15 ( ST:15 -1 (die effect) +1 Int modifier).If a magic-user wants to offset some of these steps, they can trade hit points on a one for one basis to reduce these step modifiers. Even if the spell fails, these HP are lost until recovered naturally through rest.
Example: Bryan's battle is going badly and he needs a strong spell to turn the tables. He bumps his fireball spell up to 8d6, that incurs a -3 modifier to his roll but he also wants it to hit all four of the enemies before him, bumping the AoE up to 4 and incurring another -3 for a total of -6 on his roll. He would need to roll a natural 20 to succeed (natural 20s are always a success). Not thinking Lady Luck is on his side, he opts to cough up 5 Hit Points, nearly all of his 7 HP, and rolls the dice. He needs a 15 on the roll to cast it successfully.Magic-Users can also reduce the ST number to successfully cast a spell by using this method (instead of reducing the negative modifiers). Every HP 'spent' would reduce the ST by one.
Using Saving Throws produces a naturally progressive increase in the mage's ability. Offering up the chance to cast spells more frequently, offset by the chance of failure, puts the Magic-User on the same footing as a Fighter. Both now have a change to hit or do something each round but both have a chance at success and a chance at failure.