Just for fun, I put together an idea based on how I would fit RDM into FFXIV.
I heavily based my ideas for this class on the Harbinger soul from Rift.
Base Class: None (it is its own Job just like the Heavensward Jobs)
Weapon: Sword
Cross class: Conjurer and Thaumaturge
Released: 4.0 (Far East expansion)
Role: Melee DPS Disciple of Magic
Lore
Centuries ago, the practices of White and Black Magic were commonly practiced among mages. But the misuse of these arts in warfare resulted in an aetheric imbalance that led to an Umbral Era, and subsequently the arts became forbidden.
Although many people dismissed it as a myth, stories tell of a Far Eastern cult of this time period who advocated the middle, balanced path between Black and White Magic. Calling themselves Red Mages, these formidable monks embraced the synergy between White and Black Magic, as well as the arts of the sword. Adventurers who have mastered both the White and Black can learn this long-lost art from the last few remaining descendants of this legendary sect.
Abilities
Red Philosphy: Passive ability that allows physical RDM Job abilities to benefit from Intelligence
Spellblade's Strike: Basic 150 potency melee attack
-a lines of spells: For differentiation from the Conjurer and Thaumaturge abilities, RDM will use Fira, Thundara, etc. much like FFXI's Geomancer. These will be similar to Ninjustsu abilities but act as independent castable Job spells. Cura will be used for self-healing.
Chainspell I-III: Passive ability, allows your melee attacks to have a chance of making your next -a spell instant cast (this will be used instead of combo actions)
Doublecast: Allows you to cast the next two -a spells instantly without having the Chainspell proc.
Spikes spells: Cause an effect whenever you are hit with a melee attack, depending on the element (for example Blaze Spikes may do damage, while Ice Spikes slows movement speed0
En- spells: Buffs your weapon with elemental energy for added damage and other effects
Dia I-III: Light based DoT ability
Phalanx: Scathe-like instant spell
This is just a rough outline, but this is more or less how I personally would design it. I also would like the idea of it coming in a Far East expansion, as this could be an interesting avenue for other future Jobs (such as Samurai). It would also shed some more light into the Au'Ra race.