Since the new prestige enhancements does not have any feat prerequisite, the value of Greater spell focus for off-schools kinda dropped in value a bit. The first spell focus is still a requirement for a Magister enhancement, so, I would like to suggest to add some lateral benefits over the Greater and Epic spell focus feats.
Epic Spell Focus: Stop being single school. Requires any Greater Spell Focus, and grant +1 DC to all your spells. This does not increase the power on your primary school, but reduces a bit of the gap between main school and off-schools.
Greater Spell Focus: Abjuration: +10 Caster Level and Maximum Caster Level for Break Enchantment, Dispel Magic, Greater Dispel Magic, and Mordenkainen's Disjunction (So those spells get a chance to dispel elite mobs buffs). Each abjuration buff you cast also provides the target with +1 AC and +1 PRR for the same duration.
Greater Spell Focus: Conjuration: Adds Augment Summoning benefits only for summons. Stacks with Augment Summon. Summon and cloud spells also gets a faster casting animation (quickened speed, but no quicken benefit. Quicken speeds it up even more)
Greater Spell Focus: Enchantment: Adds Augment Summoning benefits only for charmed mobs. Stacks with Augment Summon.
Greater Spell Focus: Evocation: (No idea for thematic non-numerical buff, since this is a damage-based school)
Greater Spell Focus: Illusion: Your spells also creates -20% threat than normal.
Greater Spell Focus: Necromancy: (No idea for thematic buff, but necro is a popular school anyway. It does not need one)
Greater Spell Focus: Transmutation: Each transmutation buff you cast also provides the target with +1 AC and +1 PRR for the same duration.
Mostly, those would not be game-breaking changes, and might be decent enough for some people to cogitate alternate schools besides the three major ones.