Definitely a dps with utility. Too much identity lost fitting into healer or tank roles. I'd build them on INT, using their own gear types (high mdef, mid pdef) with the option to use caster gear, with melee limit break. Weapon is a rapier. I want them to have a small shield, but that extra defensiveness is kind of unfair to other dps. Then again, they can be squishy when using caster gear and might need the protection of a shield.
They'd be a mix of a lot of existing jobs, but primarily blm and whm. All their white magic would be from cross class skills, and Cure would be moderately effective, about 5x better than a blm/smn's physick, but still weaker than healers cures, Their raise would work in combat. I'd prefer this healing prowess come from traits, but a sort of reverse cleric's stance could work too.
Their black magic would be analogous to Fire I, II, Thunder II, and Blizzard I, II, and one additional DoT spell. They get untraited sleep, and a combo enfeeble somewhat like lethargy. They would use Astral fire and umbral ice up to level II. This makes their offensive magic pretty weak, however they mainly use it to close combo's where it gets potency bonuses anyway. They also double and chain cast with cooldowns (doublecast is like duality for spells with a lower timer), while Chainspell is for openers. Rather than transpose, they can get full Astral conversion at the start of a fire combo (melee part), and have an ice/casting stance that does umbral conversion.
Combos are melee > melee > spell. Fire combo forks after the 1st hit and can either be single target or aoe. Thunder combo is single target. Cycle is basically a Thunder combo and 2 fire combosme then fire spells to use up mp, then ice/casting stance for a bit to recover resources and renew DoTs, repeat. Much like a blm's element cycle, just modified to be use melee combos that close with a spell. Rdm will need short cooldown jump into and out of melee range moves.
I'd work their enspells like aetherflow mixed with NIN poisons and call it spellblade. Each combo can change the spellblade element. All elements give a melee damage boost. Fire gives temper status (more damage boost). Thunder combo recharges a stack. Stacks are used for special attacks/spells. Some of these change their effect based on which spellblade element the RDM is using.
I call the casting stance Cold-Blooded. It switches to umbral ice and also switches spellblade element to ice, which gives Fast Cast status (cast speed boost). Magic damage is boosted 30% like mch/BRD caster stances, and auto attacks are disabled. The caveat is it only lasts as long as umbral ice is maintained, which means no fire spells and no melee combos. You can only cast ice, thumder, and utility/healing spells or you lose the stance. Even with damage and speed boosts, the damage is low spamming a Blizzard I equivalent, so most of the time you would go back to comboing fire and Thunder as soon as your resources are recovered
Rdm would have a lot of utility and some self defenses. This would included inflicting vulnerabilities on a target to increase party damage (similar to BRD and MCH), single target mp recovery, special damage shields that are weak but don't get used up until their duration ends (phalanx), and a sword drain skill/cure III equivalent (long cooldown, off gcd) for healing the front line.
