Anyway, the idea was really simple. What if, stacked metamagic multiplied an effect rather than added to it.
That's it? That's all there is to it? Wow.
eg. 3rd-level Fireball with Empower Spell x8* (+16 levels for a 19th-level spell) would deal: 10d6 x16 (10d6 x2 x2 x2 x2) = 160d6 damage.
*With Metamagic Freedom.
Instead of 10d6 x5 = 50d6
CR is right - it would be 256d6. For a 19th level spell, the cap (if we're using caps) would be 50d6 (when I posted that quote, I was wondering where that had come from, but now I know). Course, with the existing epic system (or even the one I use), damage caps pretty much go out the window anyway, but the epic system has a sort of built-in balance, in that getting it that high jacks the DC waaay up (making that spell as an epic would be somewhere around DC 510, before mitigation).
And speaking of mitigation: you mentioned you have mitigating factors. It'd be kind of interesting to see how you use those with a system that would work for all levels.
Now if we apply the Widen Spell feat to Greater Fireball, the area will double.
Fireball (3rd) = 20 ft. radius
Greater Fireball (or Widened Fireball, both 6th) = 40 ft. radius
Widened x2 Fireball (9th) = 60 ft. radius (if a double, double equals a triple)
Widened Greater Fireball (9th) = 80 ft. radius
You know, I've always wondered what math system WotC was using, where a double double equals a triple instead of a quadruple. This example amply illustrates the point, especially with CR's comment about save DCs. Why make a really big 3rd-level fireball when I can make the same size fireball at 9th level, using the same slot, and have a better chance of doing damage? The only reason people don't make greater fireballs is that it's silly - you can do the same thing with a feat.
I don't think it can be made to work. I was drawn into this fools errand because the system actually functions well up to about 50th-level, although after that it really starts to skyrocket. I must have been designing this one wearing rose tinted spectacles.
Do you mean 50th character level or 50th spell level?
I noticed that you're using damage spells as examples. What about the odd spells? Blowing up a planet is all well and good, but epic magic is about more than that, IMO. How would I, frex, create a tesseract using your system? Or transform the caster into a lich? Or wrench a castle from the ground and make it fly? These are harder to adjudicate simply using metamagic feats.