Since I'm doubting Soro will finish his Epic in Pre-epic thread, consider this the place to talk about all things epic casting related.
0a. 10th level and up spell slots granted via the Improved Spell Capacity feat aren't epic spell casting per se
0b. The Epic Spellcasting feat allows you to "develop and cast epic spells" but, for now, to keep the discussion simple, let's assume the feat wasn't necessary and that no epic feats existed. We can tackle the feats later. We're just talking about epic casting itself right now.
1. You can "cast epic spells [...] a number of open epic spell slots per day equal to one-tenth his or her ranks in the Knowledge skill appropriate to the spell and the caster’s class." They are Knowledge (aracana), Knowledge (religion) and Knowledge (nature). The above feat mentions that these stack, but DMs are likely to require fully progressed multiclassing before the second knowledge skill gives additional slots. Normally that would be hard for a Wizard 17 / x 3 character, although specialized builds like an ur-priest / blighter could manage 4 rather than 2 slots. If you want to be rule-lawyery to gain the 5th and 6th slots, remember that by throwing out (or giving for free) the epic spellcasting feat, a monk could also gain epic spells.
2. "Metamagic feats and other epic feats that manipulate normal spells cannot be used with epic spells." If epic feats can't even modify epic casting, regular feats surely can't. You'll notice the x2 for extending spells rather than being able to apply a wimpy extend spell feat to a powerful epic spell. Suddenly all of your old, high level casting tricks don't work anymore. It reminds me of people glossing over the fact that artificers aren't spellcasters. A lot of the power actually comes from those old patterns that don't just work now.
3a. Custom Epic spells should have no "guesswork and rules stretching" and should always have the "worst of all worlds" rather than the caster just choosing whatever is best. This means that all epic spells, rather than just some, have all the spell schools associated with the seeds.
3b. The Range, Targets, Area, and Effects lines all chose the worst value, additively. Nothing is stopping you from changing the target and area line before combining each seed so that they still make sense. It just costs more spellcraft.
3c. Similar to SR, saving throws are all added separately, allowing multiple instances of one save or even many instances of multiple different saves.
3d. Seeds still require a save if it deals damage, in addition to requiring a successful attack roll.
3e. Touch or ranged attack seeds still require an attack roll if it deals damage as a targeted seed, in addition to requiring a saving throw.
4. There is no recording of epic spells. You will always pay to learn/develop both existing and custom epic spells.
5. There is no additional participant mitigating factor. You can't chain gate solars for circle magic on crack.
6. Dirty-trick fix the seeds: the Armor, Conjure, Fortify, Reflect, Summon, Transfer, and Ward seeds have certain effects removed (see below). This is either to prevent TO things like loops, or because they replicate an effect already banned in any serious, balanced game. Specially they are, in order,:
Item Alteration [ECS113],
"attack of the clones,"
item alteration again and rules consistency,
pure balance so your monk ally doesn't feel even more useless when you say to the DM "I don't need delay death, I'm just immune to all attacks,"
no summoning Asmodeus,
no becoming Asmodeus,
and similar to the immunity to attacks but also with a "no coming near and no save."
The Epic spell factors you will want:
+20DC for a 1-action casting time on in-combat spells.
+25DC Contingent on specific trigger.
+4DC No verbal or somatic component for in-combat spells.
+2DC Increase duration by 100% for buffs.
+2DC Range Increase range by 100%
+10DC Change from target to area (pick area option below)
+4DC Change from touch or ranged touch attack to target
+2DC Increase spell’s saving throw DC by +1
+2DC Gain +1 bonus on caster level check to overcome target’s spell resistance
+2DC Gainter level check to beat foe’s dispel effect
+10DC Increase damage die by one step (d20 maximum)
Custom mitigation factors that are worth it:
Everyone is going to select +20DC on in-combat custom epic spells since feats & MM don't apply to epic spells.
Everyone is going to select +10min casting time for -20 DC on buffs
Everyone is going to select 2*HD d6 backlash for -2*HD DC on instantaneous spells. Share pain/vigor is assumed.
Quick Notes on the Seeds:
I'm ignoring effects that require 200+ spellcraft checks. See costs below for why.
Afflict = [M-A] lame bestow curse
Animate = Objects
Animate Dead = 20+3HD/+2DC allows ghosts, vampires, etc
Armor = 24hr 4+1armor/+2DC or +1defl,divine,insight/+10DC. ALLOW ONLY defl, divine, or insight. NOTHING ELSE
Banish = DC27 will, SR, 75' 14+2HD/+DC. Non-outsiders +20DC
Compel = [M-A] 20hr suggestion
Conceal = 3hr greater invis except CL check to see if true seeing works.
Conjure = major creation. REMOVE NEW CREATURE CREATION OPTION
contact = telepathic bond or sending
Delude = 12,000' range full illusion. Programmable, etc
Destroy = 12,000' range disintegrate. Fort save for half damage
Dispel = 300' DC19+dispel boni. Uses d20+10+ 1/+DC vs DC11+CL. Dispels anything or supresses for 1d4 rounds
Energy = energy damage
Forsee = CoP @90%
Fortify = DC47,DC215 (23+4*6)x5-20 permanent +5inh to stat, save, nat or DC115 for permanent SR25 or DC32+2x for 1+x DR/epic. ALLOW ONLY enhancement & inherent BONI. DO NOT ALLOW BONI TO STATS YOU DON'T HAVE
Heal = 100% hp heal+restoration or fort or 1d4hp currently via negative/positive energy
Life = ressurection or awaken anmial/plant
Reflect = spell turning (linkens' sphere). REMOVE THE MELEE AND RANGED ATTACK REFLECTION OPTIONS.
Reveal = scrying w/ true seeing
Slay = [Death] fort for 23-38damage or enervation +4DC / +d4 negative levels
Summon = 20round gate w/o dominate. REMOVE THE INDIVIDUALS OPTION
Transform = permanent metamorph w/ gas&incorp. REMOVE THE INDIVIDUALS OPTION
Transport = teleport+plane shift or time stop
Ward = 24hr 18+2*x DC 5+x DR or an energy res or a spell immunity. REMOVE THE HEDGING AND NULLIFY OPTIONS
On specific spells and seeds:
Spoiler (click to show/hide)
Not that it matters, but I can't see Time Duplicate being usable due to time paradoxes: cast the spell, and then quicken kill yourself. Did the spell text lie? Did you just break the game and the metagame simultaneously?
Animate dead can get you ANY type of undead. It RAW forces x kind of undead to appear before you. Although its not exceedingly powerful, I can't think of any other effect that does this. Therefore it's balanced and could be useful.
Dispel = 300' d20 (+any other dispel boni) to see if its less than CL. Can 1d4 supress ANYTHING. Free @11min
Armor = 48hr +6armor for free 11 minute cast
Ward = 48hr DR5 for free 11 minute cast
Costs:
Ironically the above seeds should be used without any other seeds that would make the spellcraft checks higher. If you take a DC20 epic spell with a -20DC modifier, you can learn the spells for free. Spending any more is a waste. Otherwise you would spend 9,000gp of per extra spellcraft DC. That quickly adds up, especially when you remember that the archetypal full caster has invested in some serious crafting reductions in order to stretch his WBL. But since none of your tricks work on epic spellcasting, you have to shell out the gold the old fashion way. Remember that with lazy amounts of crafting modifiers, your WBL is about 10x the number listed on the DMG. So without those, the epic development costs basically have a 10x price tag on them. A +1DC isn't comparable to a 90k non-epic item.
Basically unless you have huge piles of gold laying around, which you shouldn't if you are using your WBL wisely, it seems to me that you are better off putting those resources into non-epic areas where you will get higher return on investment. I am interested to see if someone pulls together a custom spell with the above restrictions that's worth spending a third of your ECL20 WBL on.