Sepulchrave II
Legend
I think a wide selection of arcane-focussed epic feats can help address part of this problem.Don't forget the notion of clerics using their turning attempts to power epic spells!
I thought, for some reason, that epic wizards got more feats than epic clerics. But checking at http://www.d20srd.org I see this is not the case. Perhaps only arcane classes could be allowed to spend class-specific bonus feats on improving their epic magic?
Druids aren't so hot as epic casters (1 bonus feat/4 levels), although arguably their other class abilities more than make up for it. True Spontaneity was originally conceived to give the epic sorcerer an edge - an Impulse, or impromptu epic spell. Still vague about the implementation, though. Likewise with the Magnum Opus for wizards. The specifics of these feats are still numinous, though - their inspiration was as a counterpoint to the benefits offered by divine seeds and [divine] feats for clerics.
I know that the fact that they require a feat to take should not necessarily be weighed against a class benefit. But the fact that the caster has to take a feat to reap the benefits should make them very bloody powerful.