Hmm If we are going to go with Off-hands I would just say for Bard the the second equip should be Arrows or a quiver. Add elemental damage,stat damage, stuff like that.

I am not familiar enough with Dancer or Machinist, but I think all dual wield type of DPS should be able to choose the second weapon. One chakram is ice the other is fire?

Ninja- 2 different daggers.

Red Mage- Blade is separate from the orb thing.

Maybe a book with a bookmark or a quill SMN/Scholar?