Thing about FFXI's RDM is it had tons of things working against it, here's a few that are just the tip of the iceberg.

-Magic and Melee did not work cohesively like with BLU
-Staves became required due to their benefits pretty much overshadowing anything RDM got from Swords and Daggers, and since swapping weapons meant lost TP it threw weapon swapping out of the equation.
-Stagnation, oh Lord did they let RDM grow stagnant. No unique Enfeebles outside of merits and lolGravity II, all new buffs are self-buff, no worthwhile new spells or abilities to contribute to party play, everything worth a damn is obtained through sub-job (yes literally everything).

But there are some things that FFXIV would bring to RDM that would help deter the follies of FFXI's RDM.

-Armoury System, means we'd wear a fencing or rapier type weapon full time.
-The limitations on the amount of abilities and weaponskills obtainable would give way to some creative ways to combine aspects of RDM such as WS>WS>Spell combos or Magical WS combos (Think Stiner's Magic Sword abilities from FF9).
-The power of spells and abilities scale with your level, effectively eliminating the need to have more than a few tiers of magic and the BS skill system that plauged FFXI's RDM.
-RDM technically is somewhat pre-designed with pre-1.19's Thaumatarge, and could easily benefit from using some of the removed spells like Scourge, Banish, and Sacrifice, or using some of them as a basis in the design for RDM.

But I digress, the limitations with the Armoury system are actually a good thing in the end, it restricts what you can really do with jobs and classes, so you have to think with job/spell slot limits in mind and how that would work for or against a job. Like if they just gave RDM a Single WS combo, Single Magic combo, and a few spells here and there it would probably be Underpowered not being able to dedicate itself fully to a role. Now if you gave RDM a three or four Magic WS combos or WS combos with a spell finisher or a stance ability to turn your magic WS's into long ranged nukes that might put RDM in a bit more lovable position even if they scale back the raw damage of their abilities and spells a little.