Final Fantasy Dimensions didn't have Mystic Knight or Rune Fencer but it had Red Mage with Spellblade, which had the skills you'd associate with the former. Ensorcelling your sword similar to how NIN does with poisons to modify the additional effects of your oGCDs could be neat for RDM and I sincerely hope they add the Enspells to RDM considering they're in the game and many, many mobs use them already.
