I would just start with a new seed based on Miracle.
Seed: Miracle
(Evocation)
DC: 31
Components: V, S
Casting Time: 1 Minute
Range: As Spell Duplicated
Target, Effect, or Area: As Spell Duplicated
Duration: As Spell Duplicated
Saving Throw: As Spell Duplicated
Spell Resistance: Yes
A miracle can do any of the following things.
* Duplicate any cleric spell of 8th level or lower (including spells to which you have access because of your domains).
* Duplicate any other spell of 7th level or lower.
* Undo the harmful effects of certain spells, such as feeblemind or insanity.
* Have any effect whose power level is in line with the above effects.
A duplicated spell allows saving throws and spell resistance as normal, but the save DCs are as for a 10th-level spell. When a miracle duplicates a spell that has an XP cost, you must pay that cost. When a miracle spell duplicates a spell with a material component that costs more than 100 gp, you must provide that component.
To increase the cleric spells duplicable to include 9th level, add +2 to the DC.
To increase the spells of other types castable, add +2 to the DC for each level.
So an Epic Miracle that could duplicate any spell of 9th level or lower would be DC 39, and have a casting time of 1 minute. Mitigate to taste.
I'm not sure how balanced that is in play, because it opens up all kinds of wizard spells and so on, but you're using an epic slot to cast a non-epic spell. Whoopee. Maybe +5 for each non-cleric level castable? Dunno.
That's how I'd start going about it, anyway.