Question, UK: are you going with spell slots that you automatically gain (like with 0-9) or with AMC slots that you have to blow a feat on? I've been doing some work on a new, simpler, epic system (I posted the basics in the House Rules forum about a month ago, which means it's probably gone now) and I found that you can gain 1 new spell/3 levels without breaking the system (the same as feats, really...).
And will you be able to put epic spells into artifacts? That's one of my many gripes about the "epic" spell system - you can't put them into magic items or artifacts. WTF?
I was thinking about this earlier today. Ten applications of widen (at +3 levels each) and the ddq rule will increase all dimensions of an effect by 1000. Disintegrate will make a 10 foot cube of matter disappear. Times by 1000 is about 2 miles on a side. Another 1000 and you are talking a 2000 mile cube. A sphere 2500 miles in diameter has about the same volume. Mercury's diameter is about 3000 miles; Mars's is a little over 4000 miles. So 20 doublings would be enough for a small planet, and 22 would disintegrate the earth. Assuming it failed its save. I forget what the saving throw bonus is for a planet.
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According to this chain of reasoning, anti-magic entire planet's surface should be 30 something levels higher than antimagic shell. So about 40. And destroy planet should be 40 something levels higher than disintegrate. So about 50.
Something else I've been tossing around - I call it the Principle of Bulk Consumption. When you buy things in bulk, you get them cheaper, right? So why shouldn't this apply to metamagic? The rule is this: If you apply the same metamagic feat to a spell more than once, the level adjustment for each application is reduced by 1, to a minimum of 1. So, the spell Cheiro's suggesting, to surround the earth with an antimagic field (using my system)would be: antimagic field (L6); widen x22 (+44) = L50 spell.
I really like the idea of DDQ as opposed to DDT - it makes a lot more sense from a mathematical POV, and it's a lot easier to figure out larger areas and such. I think I'll use that for mine.
One thing I'm having difficulty with is enlarging effects greatly- how do you turn a duration of rounds into a duration of hours, or a 20 foot radius effect into a 2000 foot radius effect? Surely not by piling on 600 iterations of the extend feat, or 99 iterations of the enlarge feat.
You could just make a (possibly epic) feat that increases orders of magnitude as opposed to iteration. For example: Epic Extend Spell (okay, so my names suck

) - for a cost of, say, +5 levels, you can increase the spell's duration by one step, from rounds to minutes to hours to days.
I know why it was put in place, I was reading every article and designer interview I could get my hands on around the time of the 3E release.
Well don't keep us in the dark - speak! Enlighten us, so that we may know the inner workings of the great minds at WotC.
Another idea may be to have Int bonus set as the 'Epic Spell' Prereq. rather than just 10 + spell level it would therefore be 10 + 2/spell level of an epic spell.
I hope by "Int bonus" you really mean "spellcasting ability bonus", otherwise all other casters are boned. And it's ability stat = 10 + spell level, not bonus.
While changing it to ability bonus = 10+1/2 spell level might seem like a good idea, I don't think it is. A L9 spell requires a 19 stat, while a L10 spell would require only 15, feats notwithstanding. What if the character gets ability-drained? He loses the 9th-level spells, but can still cast 10th+? No way, dude. So it requires an absurd ability score to cast a spell to move the galaxy - how often is someone going to cast something like that anyway? Once every million years?
Heres a point to discuss: With the printing of Champions of Ruin (Forgotten Realms evil suppliment), they list that for a +25 increase to the DC, a single factor of the spell, like range, or damage, can be based on Caster Level. (So if have a spell that does 20d6, and add 25 to the DC, it does 20d6/level. )
Is this totally broken, or is it just what the system needed?
I like it, but it screams abuse.
I used it for the level-based system, at +4 levels, and it worked fine. Their version (if what you say is correct) is hideously broken, however. If you set damage to scale by caster level, it should be 1d6/level, not base damage/level.
(xp formula is 100*n*(n+1)/2, where n = spell level - 9)
This is only for spells that use "real doublings" instead of the "two doubles is a triple" kind of D&D math, right?
Is this for XP cost, or the XP require to develop the spell? For that matter, UK - does your system follow the ELH, where developing epic spells requires XP as well as gold?
And finally:
Here are a few things I posted many months ago about balancing the (epic) spells above 9th-level.
I think if you up all those by 10 levels, you'll be closer to the mark. Like I pointed out before, a planet-wide antimagic field is ~L50, and someone else (Cheiro?) said that creating a star would be L70ish; teleporting a city would be around ~30. Assuming it's around 1 mile, we take teleport object (L7), change from target to area (20-ft. radius, +7) and widen x8 (+16, for an radius of 5,120 ft.), we end up with L30.