RDM can definitely fit the support dps archetype.

Make EnElement spells buffs that improve specific archetypes, so EnFire deals additional fire damage on spellcasts, EnThunder deals additional lightning damage on skillcasts, EnQuake deals additional earth damage on skillcasts and reduces damage taken for a while(damage lower than Lightning), EnLight causes heals to heal more.

Of course, there's also Dia, a defense shred. Refresh an MP version of Goad and Raise.

And if it was up to me, I'd also allow them to cross class Cure and Medica. But due to low MND these spells would need to be Doublecast to be really effective. And Dualcast would be the real crux of the RDM class, seeing as it's their signature spell in the series.
As a twist, I'd have dualcast be applied as a buff when you use a basic three stage combo. So you'd go 1 -> 2 -> 3 -> get dualcast -> Fire as your basic damage combo. or 1-> 2 -> 3 -> Dualcast -> Medica if you want to help aoe heal.

Dualcast casts a spell twice, with the second one costing no MP and dealing full damage/healing. Effectively doubling the spell's effect.

Main stat intellect. And intellect increases both magical and physical damage.

Of course, none of this will actually happen. But I'd play a class like that, even if I have to suffer through leveling with extremely subpar personal dps.