I agree that RDM needs some twerking to get put back on the table in terms of end-game, but as a mostly casual player (I still do plenty of endgame with a shell, but I ultimately log in to relax), I would like to see more canonical RDM like abilities and spells.

X-Magic / Doublecast / Quadra-magic
This was always a cool ability that sparked my interest in the older FF games in the series. While I understand RDM isn't the best nuker to begin with, this would still be a fun ability to be had. I ASSUME (that means I don't know the mechanics of the code that is used and how this could be implemented. Just putting that out there before people start yelling about it.) that it could be implemented just like Double Shot for RNG. Perhaps even a double-attack trait for magic that has a percent chance to proc.

Melee
I know what you're thinking. RDM + melee? luluzluzlulzulz. No. RDM is a swordsman as much as a PLD is a swordsman. They should definetely have a buff to their physical prowess. Whether this is through a buff to enspell damage/effect or through straight skill increase.

Parry
They're a swordsman, and so typically a swordsman would parry. I think it would be pretty rad for them to get a trait that ISN'T dual wield, but allows them to equip a parrying dagger in the off-hand. Think of it like how a strap functions for 2-handed weapons. The off-hand dagger would not attack, but greatly increase the chances of parrying.

Fast-cast
RDM is supposed to be the king of fast cast. Quick magic could be a trait native to RDM to compete with SCH's abilities. I understand some jobs can use equipment or atmas to utilize this, but a RDM could have a much higher proc chance with this.

Enspells
Since my Mystic Knight idea was kind of thrown in to Rune Fencer, I think RDM would benefit from En-spells with different effects. At the very least a 3 minute or so ability that adds an additional effect to en-spells along the lines of Enstone: adds additional effect Slow, Enthunder: additional effect Stun.

Crowd control
RDM could also benefit to have more crowd control resources. Perhaps an improved Bind. Old school enfeebles like Confuse. An intimidation spell, or a spell to lock an enemy's focus on the RDM while they kite it. (Old school Kirin kiting ftmfw).

Okay, I forgot to submit this earlier this morning. Forgive me if someone else happens to mention something like I have. Probably shouldn't get in a FFXI forum at work.