Following on from the previous post, let's try and extrapolate the math for weapon dice onto spell dice.
Now, the distinction would be that spells are expected to use either INT or CHA, but if they're using the same actual dice as weapons, then the modifiers might necessarily need to change. To wit, if the assumed "standard array" is +3/+2/+2/+1/+1 but a two-weapon routine is not assumed in the spell math, then we'll need to tinker a bit in order to create something comparable to the weapon dice math.
As an aside, I think that 4e D&D at least sets the bar for how we should think about spell damage. For example, a "close burst 1" attack would hit all adjacent enemies (for all intents and purposes.) Since we're talking about miniatures on a square grid, this means a 3x3 square, excluding the center square; in short, a maximum of 8 enemies could be targeted. Likewise, strategically, it might be reasonable to assume that a power such as this would not be used unless, say, there were at least 3 enemies adjacent. So in the paradigm of 1dX+mod, this gives us a range of that number, multiplied by [3,4,5,6,7,8]. You can easily see how multiplying (in particular) the modifier so many times can lead to massive spikes in the expected damage curve. Generally D&D (especially outside of 4th Edition) 'tries' to "balance" this by making such AoE spells limited on a per-day basis -- in other words, they make it apples and oranges, and hope nobody audits the math too closely.
If we're building from the assumption that spells are (generally, mostly) ranged attacks, then it would make sense to try and have them be at parity with ranged weapon attacks. In the previous post, we established that a 1d8 ranged weapon attack would add the higher of STR or DEX -- so a 1d8 ranged spell attack should do the same, albeit with INT or CHA. With a +3 mod, this gives an average damage of 6.8 (accounting for the d8 attack bonus math.)
Now, if we take an unmodified d4 attack and allow the spellcaster to make 3 of these per turn (i.e. a magic missile spell, that does "a number of attacks equal to the higher of your INT mod or CHA mod") this comes out to a total of 6.6375 average damage, including attack bonus. Already we can see that this damage almost exactly on pace with the d8; granting more than 3 attacks is probably not in the cards. (If anything, we might be able to do fewer attacks, but attach a small damage modifier to them.) It's probably worth mentioning here that 13th Age tends to use "1d3" for determining the number of targets on multi-target spells, and d4 serves this function in TNP sometimes, for similar reasons.
Finally, a d12 attack with a +1 modifier gives an average damage of 6.45, or a 7.15 average with a +2 modifier. This math is pretty easily attainable if we use our previous mechanic for the d4 two-weapon fighting, whereby the die uses the lower of STR or DEX (in the case of a spell, one assumes INT or CHA).
So, we can see already the framework starting to take shape. We've made attribute modifiers an important part of the math, but kept things relatively balanced within those parameters. I've always sort of liked the idea of having weapon dice function as spell dice; with daggers typically being d4 and staves being d8, I think this kind of thinking maps well to this sort of dice paradigm.
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A relatively short post today, but most of the groundwork was laid in the previous one.
Following post is due on Nov. 20th so check back then!