A Disciple of Magic who uses one of the three melee DD gear sets could be easily differentiable from RDM by being the opposite of what a RDM is: a mostly melee DD who occasionally casts ranged spells. And even though elemental damage means nothing mechanically, the various elements can still be present in their attacks by providing various effects similar to how BLM doesn't deal elemental damage but the elements have meaningful effects on their job mechanics and rotation. Bonus points if they actually make significant use of the Water element in their action set because nobody else does.
In terms of Mystic Knight in particular, one idea would be for them to have a normal 1-2-3 melee combo that does pitiable damage until it's imbued with an element via an oGCD. E.g.,
- Fire Blade - Melee weaponskills deal (1.2x, 1.8x, 2.4x) damage but cost 10% MP per weaponskill (Addendum: As with BLM, native MP refresh drops to 0 while under Fire Blade. Three full Fire Blade combos should use 90% of MP, leaving MKN with 10% MP.)
- Blizzard Blade - Melee weaponskills deal 1.0x damage but
the next three weaponskills restore 20%,30%,50% MP, respectively. (Edit: the next three weaponskills restore MP. Each restores 20% if used out of combo, step 2 combo bonus restores 30% instead and step 3 combo bonus restores 50%. So you get as little as 60% MP back if you screw up your combo.)
- Thunder Blade - Melee weaponskill chain deals 1.0x damage but apply a DoT to the target. DoT stacks up to three times, maybe.
- Drain Blade - Melee weaponskills absorb some of the damage dealt as HP, but some use-limiting criteria needs to be set so this can't be kept up for more than one or two combos.
Then they would be melee DDs whose rotating elements have the MP bar going up and down in a way that feels very familiar to BLM players. And more powerful, build-up-to-these imbuements like (Burst/Flare/Flood) Blade might be Spells or Weaponskills on their own, imbuing the weapon and performing a finishing strike in one action. (Edit: Flare Blade could be a finisher that uses all MP under Fire Blade, expected to consume the last 10% left over after three weaponkill chains.)