as stated by yoshida, they can modify the class skill for fit the role of the jobs added to the second jobs of any class.

let's take the example of the pugilist and conjurer.

for the conjurer, it seem pretty clear that the second jobs will be geomancer (mostly because the skill are based on the element and in french the class is called elementalist) they can make all the light spell become water spell when you become conjurer and even add an element type wood from them. it only recquire to change the healing capacity into an offensive capacity or debuffing one.

for the pugilist, i think they can introduce the jobs of dancer, using the stance as the mouvement of dance, the combo system can be changed for fit into this, instead to attack skill of pugilist you will have step of dance, leading to some buff or healing done, i can perfectly imagine this and how it will be really interesting as gameplay.

they are free to make anything since this statement from yoshida, but actually i think they will add what we miss before add second jobs from the actual class present. what i means by what we miss?
- dual dagger user (or dagger)
- 2h sword user
- gun user.
- mace/2h mace user

from Final Fantasy viewpoint or even mmorpg, the lack of dagger and two hand sword feel really strange. and before you say that we have two hand axe, they are not used in the same way ^^