One of the first things you'd need to do would be to add a stance (or trait) that allows you to use DEX instead of STR for melee attacks, probably at level 2.
Reveal would need to only apply to piercing damage to prevent *massive* value inflation. An improved Reveal trait later on (40-50) *could* have it apply to more than one damage type, but not all.
I would also only make one of the "combo spells" a baseline functionality of the Fencer. As you said yourself, the Fencer shouldn't be hugely into magic. One variation might be to have Magic Sword act somewhat like Defiance insofar as it adds bonus functionality to existing combo attack (this is actually something I want to go with; I know, I'm so fickle in design, but that's what happens when you do the brainstorming thing).
Also, I'm going to avoid applying levels to everything. That just gets wonky and hard to organize. The gear construct I'm envisioning is a rapier in the main hand and a main gauche (parrying dagger) in the other.
There are 17 "slots" for class abilities, 11 slots for class traits, and 5 slots for job abilities. None of them are presented in the order in which they would be received.
For later reference, the class would be treated as an mDPS class by default for purposes of Limit Break (this will make sense when you get to the job abilities).
Class Abilities:
1. Primero: 150 potency attack
2. Secundo: 100 potency attack, Combo: Primero, 200 potency
3. Flourish: 100 potency attack, Combo: Secundo, 220 potency
Freecasting: Attack deals fire damage and further fire damage over time for 18 seconds, potency 20 (Burn)
Magic Sword: Attack deals fire damage and further fire damage over time for 18 seconds, potency 20 (Burn)
4. Weaving Strike: 100 potency attack, Combo: Secundo, increase parry chance by 20% for 12 seconds
Freecasting: You and all allies within 5m gain an absorb shield, potency 100
Magic Sword: You gain an absorb shield for 10 seconds equal to 300% of damage dealt
5. Lightning Thrust: 100 potency attack, Combo: Secundo, 260 potency
Freecasting: Attack deals lightning damage and 15% chance to paralyze target for 5 seconds (Shock)
Magic Sword: Attack deals lightning damage and 15% chance to paralyze target for 5 seconds
6. Hamstring: 100 potency attack, Combo: Secundo, 150 potency and 30% heavy for 18 seconds
Freecasting: Attack deals ice damage, heavy is increased to 40% and ice damage over time, potency 10 (Freeze)
Magic Weapon: Attack deals ice damage, target's attack speed and cast speed are reduced by 20% for 18 seconds
7. Terzo: 100 potency attack, Combo: Primero, 160 potency and increases critical hit chance by 30% for 18 seconds
Freecasting: Deals 20 additional potency for each of Burn, Shock, and Freeze on the target
Magic Weapon: increased enmity (x3)
8. Moullinet: 100 potency attack, Combo: Terzo, target takes damage over time for 18 seconds, potency 35
9. Glorioso: 100 potency attack, Combo: Terzo, 280 potency
Freecasting: Recovers MP, no MP cost
Magic Weapon: Recovers MP, increased enmity (x6)
10. Feint: Increase critical hit chance by 25%, removed when you score a critical hit or 10 sec, off-GCD, 30 second CD
11. Riposte: Can only be used after a parry, off-GCD, 200 potency attack
12. Strip Armor: 120 potency attack; reduces target's piercing resistance by 10% for 18 seconds
13. Pommel Strike: 130 potency attack, stun target for 3 seconds, 30 sec CD, off GCD
14. Skewer: potency 135 attack to all enemies in 5y line
Magic Weapon: increased enmity
15. Luck Over Skill: Increase critical hit chance by 30% for 10 seconds, off-GCD, 45 second CD
Magic Weapon: When you score a critical hit while this ability is active, parry chance is increased by 30% for 10 seconds
16. Offhand Toss: 120 potency 15y ranged attack
Magic Weapon: increased enmity
17. Renew Spirit: Restore hp, TP, and MP, Cure Potency 150, 200 TP, 20% MP based off of attack power
Class Traits:
1. Enhanced Moullinet: Increase Duration to 30 seconds
2. Enhanced Feint: Increase critical hit chance to 50%
3. Enhanced Weaving Strike: Increase duration to 18 seconds
4. Enhanced Luck Over Skill: Decrease CD to 30 seconds
5. Enhanced Dexterity I: Increase Dex by 2
6. Enhanced Dexterity II: Increase Dex by 4
7. Enhanced Dexterity III: Increase Dex by 6
8. Agile Fighter: Use Dexterity instead of Strength for melee attacks
RDM Job Abilities:
1. Freecasting: Stance. While active, some of your attacks allow you to cast a linked spell. Such abilities are given an MP cost in addition to their TP cost. If you do not have the necessary MP, attack resolves without the additional benefit
2. Healer's Aura: All allies within 30y have the potency of their healing effects increased by 10% for 20 seconds, 120 sec CD, off-GCD; While this is active, your Limit Break is treated as if you were a healer, cannot be used while Warrior's Aura or Caster's Aura is active
3. Warrior's Aura: All allies within 30y have the potency of their attacks increased by 10% for 20 seconds, 120 sec CD, off-GCD; while this is active, your Limit Break is treated as if you were a tank, cannot be used while Healer's Aura or Caster's Aura is active
4. Caster's Aura: All allies within 30y have the potency of their spells increased by 10% for 20 seconds, 120 sec CD, off-GCD; while this is active, your Limit Break is treated as if you were a caster, cannot be used while Healer's Aura or Warrior's Aura is active
5. Double Time: Increase skill speed by 100% for 10 seconds, 2 minute CD
MYS Job Abilities:
1. Magic Sword: Stance. While active, you are able to parry magic as well as melee attacks and your Defense is increased by 45% and your Magic Defense by 150%. Some abilities gain additional effects. Consumes MP over time and reduces damage by 25%. 20 sec CD
2. Shield I: Usable only after a parry. Provides an absorb shield for 10 seconds, potency 300, off-GCD, 5 sec CD
3. Shield II: Provides an absorb barrier and reduces incoming damage by 20% while active, lasts 10 seconds, potency 500, off-GCD, 90 sec CD
4. Unleash Magic: Attack with potency 600, Ends Magic Sword.
5. Masterstroke: Attack with potency 300, increase parry chance by 100% and parry strength by 20% for 10 seconds; 120 sec CD
Note: MYS gear is DRG/LNC gear and uses LNC as an additional job with Keen Flurry as an additional.
Basically, there are 2 separate combo trees: the Secundo tree, which is the "magic" combo tree, and the Terzo tree, which is the standard melee tree (the mp restore on Glorioso is to replenish the cost accrued by Magic Sword). Some of the combos are intentionally situational: Hamstring is terrible damage for a tertiary combo even with Terzo's bonus damage and the DoT included. You would only use it when you need to apply heavy.
Beyond that, Fencer's theme is pretty heavily +crit oriented (makes sense since it's not using a precision fighting weapon).
RDM adds a *lot* of versatility and support to the class. I liked the idea of being able to parry magic attacks, but thought it made more sense as part of MYS than Fencer or RDM (what with MYS being the "magic tank"). The way this is set up, Fencer is still a fully non-magical class and RDM just ends up auto-casting certain spells with certain attacks. The ability to do a crapton of different things is present in the various Aura abilities. Personally, I like the idea of a single class being able to pull perform whatever limit it might need to perform at a moment's notice, and RDM would be perfect for that, what with its generalist tendencies.
Mystic Knight, on the other hand, isn't as versatile but acts as an avoidance tank. It has lighter armor than the other tanks, but the stance and higher parry make up for that. The difference in mDef is made up by being able to parry magic attacks as well. The various +parry abilities act as mitigation on everything thanks to the "parry magic" aspect of Magic Sword.
With the way I've set it up, the requirements for RDM would likely be something like 30 FNC, 10-15 CNJ, 10-15 THM and MYS would be 30 FNC, 15 ARC.
By the way, the numbers are nowhere near solid. I don't know enough about DPS potency modification numbers or baseline crit rates or anything, so most of those numbers are just there for comparison's sake (i.e. Moullinet does more damage than Glorioso but only when the DoT is allowed to fully resolve).