When I read this idea I hated it. Then I read all of the comments and thought the following:
If they implement classes to be an all-rounder family name where you have to get "[pet user class name]" to a certain level to unlock various jobs (or specializations) a la getting to level 30 in FFXI and then being able to unlock advanced classes then they could fuse this into a few areas. So for this idea the class you'd level could maybe have a few elemental spirits to control, a weak charm and/or a weak transformation based on a boss you had to beat (maybe this would depend on your choice of a sub-category of this class type's weapon). Then the jobs could be specialized in the following way:
Job1: Traditional summoner having to beat the Primals in a fight/quest and then summon them to their aid.
Job2: Non-traditional summoner having to beat the Primals (in a separate fight/quest) and then transform into a humanoid/primal blend.
Job3: Traditional beast master being able to charm/trap monsters for use as pets.
Job4: Non-traditional beast master being able to fight certain quest monsters to gain the ability to transform into or use the monster's abilities on their own means (sort of like a blue mage I suppose or possibly like that painter class girl in ff6).
So kind of like a tree. Maybe each of the jobs could go into an advancement to make the family go:
Pet Class - all rounder, non-specific, weaker(or shorter duration charms)/"jug" pets/spirits(elementals)/etc
-Job1 stronger elements than class
--Job1a use of primals
-Job2 imbue an elemental giving you an aura/spells
--Job2a transform into a primal/humanoid race hybrid
-Job3 temporary charm pets
--Job3a permanently trap pets
-job4 gain use of some monster abilities
--job4a transform into some monsters
(edit: the "a" stands for "advanced" - just thought I'd clarify)
Just my two cents. (and my first post ^-^)
Quick Edit: Sticking with FF trend though you'd be able to unlock all 4 jobs and switch between them as you desire.
Edit2: Hadn't read Lienn's post #29 so this basically just wound up being the same general idea with a few adjustments and a little more fleshed out. >.<