Here is a thought I've kicked around previously, it is the idea that each Wizard Specialist is handled by a partial caster class. For example
Evoker -> Magus
Conjuration -> Summoner
Enchantment & Illusion -> Mesmerist
Abjuration -> Magus?
Transmutation -> Alchemist
Necromancy -> Summoner who specializes in undead??
Divination -> Got nothing for this one.
Bard could fit a couple of roles such as
Transmutation / Divination -> adventuring bards
Charm / Enchantment -> City / Intrigue bards
This idea is partially in answer to the creation of a lower magic setting which is limiting magic to 6th spells or lower and still allowing for a full 20 level character progression.