Honestly what makes RDM "RDM" is the "jack of all trades" which wouldnt stand out in this game, unless it was the ONLY job that could be all 3 roles. (Lets say you're a RDM job, but instead of a "job stone" you get 3 shattered RDM stones, that give either tank, DPS or Healer "stances"/abilities.

(To be honest, Im rooting for either tank, or melee healer, but id still enjoy it as DPS)

The only thing left to giev RDM as a unique trait, would be a "Double Cast".
And again, only way I can think to do it, is a sort of "Stance" that just casts a predetermined spell alongside any other spells.
(If healer, casts a heal on your target, if tank, casts phalanx or something on yourself, if DPS casts a spell.)
And maybe you can only "double cast" CNJ like spells, while directly casting THM type spells spells.
(Somehow needing to alternate between melee and magic, to keep resources up. If a melee ability is required to keep healing, would force them into "melee healer" role slightly. (Maybe shorten the spell distance for melee heals.))