Personally I see RDM as Discipline of Magic Caster DPS with a focus on proximity and placement to mobs. A mix of Melee combos and ranged Spell rotations. A gap closer and a gap maker. Throw in a 60-90 recast CD that buffs the ranged spell drastically (but requires range away from target to fully buff the ability like BRD/MCH's DPS in PVP), so RDM use the gap maker to move away from the target and do a spell rotation then gap-close back to the target.

tl;dr: a caster that's max dps is dependent on them moving around - not standing still and turrenting (I agree with the hat comment)

Back to OP. If SE were to ever make another dual job class again like Arcanist, I can really see it being a 'Swordsman' class into Samurai and Sword Master. Sword Master is a bit of a Tactics Orge shout out but then again that's where many FF classes originate anyway.

Other than that obvious pick, Beastmaster?? That'd be a hard job to make into a tank, but man it'd be neat if they could pull that off