What if they made it so your subjob is determined by what abilities from other classes you equip? For example, if I equip 4 abilities from marauder, then I would be Gladiator/marauder. But if I only equip 3 from marauder and 1 from conj, then I won't have a subjob, just a mixture.

You would still be able to equip skills from every class but specializing in a skill set would give you a bonus to those skills.