I see the new RDM as a spellsword that uses self skillchains and magic bursts. Something like WS > WS > Spell.

Their combos would set up a temporary weakness to an element so that a nuke or enfeeble goes through with little resistance.

They could have a choice of offhand: shield or orb. The shield would raise defensive and melee stats, the orb would raise magic defense and attack. A skill would allow them to switch this on the fly, bypassing the gear swap limit in combat.

They could also self skillchain into a buff or heal, increasing its potency. However, as this is not as optimal as a true healer; they would not threaten main role healers.

Seeing as they would have an impressive list of spells possibly making them OP, I suggest a Limited Action Set like how BLU had in XI. Each job quest would unlock multiple spells, but they would have only 10 slots for spells at 50, same as base classes. This would force them to chose a set appropriate to the content.

Ideas for spells(mostly rehashed):
Enfeebles: Gravity, Slow, Blind, Bind, Stun, Pacify, Def Down, Atk Down, Dispel
Nukes: Same as THM and CNJ, minus one tier
Buffs: Haste, En-spells, Element Spikes (dmg or status reflect)
Heals: Same as CNJ minus one tier