Introduce Seraph (an esper/summon in previous titles) as a primal. Seraph brings a mighty sward with her from heaven. In the cutscenes after the Seraph Storymode fight, you can use the holy power of the sward to cleanse the evil from your order and continue as a real paladin. Your other option is to give in to the might/power of the dark and corrupt the sward along with yourself. At this point, at least if i were the writer, Paladin and Dark Knight would be mutually exclusive on the job list. This would explain why DRK didn't have a "class", because it would require an already leveled "job" to use. It also agrees with the lore from IV: Cecil had to cast aside his dark knight ways and embrace paladin. This obviously should be reversible somehow (like talking to an NPC to re-enable pld instead of drk etc) but i like the idea of PLD/DRK being unable to be selected out of the armory list without a special flagging to reflect your choice.