The new Spycraft 2.0 rulebook uses a blast increment for most "explosive" effects...  I was considering using the same mechanic for spells in D20 fantasy games, notably D&D and Iron Heroes.  Here's how it works:

Instead of a fixed blast radius, "explosive" (most damage-dealing bursts, emenations or spreads, for the purposes of D&D) effects have a blast increment.  Within the first increment, full damage is dealt.  For every blast increment after the first, damage is halved (round down).  Applicable saving throws still apply.

So, for example:

Say we convert the fireball spell so that it has a blast increment of 10', instead of a fixed radius of 20'...  A 6th level Wizard casts a 6d6 fireball, and rolls a total of 26 points of damage.  Everyone within 10' of the point of impact would take 26 points of damage (Reflex save for half damage).  Everyone between 10 and 20 feet would take 13 points (again, Reflex for half).  Between 20 and 30, 6 points.  Between 30 and 40, 3 points.  Between 40 and 50, 1 point.  Beyond 50 feet, no one takes damage.

This could not only be appplied to spells, but any other area special effects...  A red dragon's cone-of-fire breath weapon, for example.

The advantage is that it would greatly extend the area of effect, with diminishing damage.  The disadvantage is that it makes precision spellcasting much more difficult for many of the stereotypical artillery spells... There's a good chance that you could catch your friends in the fringes of a blast, dealing them minor amounts of collateral damage, especially if you happen to roll well for damage.

What do you guys think of the idea?

I would welcome this small added layer of complexity. I've always disliked the precision with which fireballs and the like can be placed.

There is a bit of added complexity which isn't too much of an issue in Spycraft since blasts don't occur as often as say when you have a Warmage in the party. If someone is dropping a fireball frequently then it does become a problem as it will slow down the game.

But how would it work with things like the Shape spell metamagic feat?

Quote from: BagpussBut how would it work with things like the Shape spell metamagic feat?

Good question...  I hadn't thought of it.  I'll have to go look that feat up.

I actually had a very similar thought just yesterday, but I was just going to make it a separate spell rather than a global change.  So you could still have a RAW fireball, or you could have an "explosion" spell.  I was going to do it like Champions explosion damage, where each hex from the center you remove one die of damage.

So at 10th level you're doing 1d6 out to a radius of 50 feet.  Maybe even make it a 2nd level spell, since I'm pretty sure it's less powerful overall, and it lets 3rd-level wizards have a spell that goes boom.