Fencer would make a good counterpoint to Dual Wield. As a person who quit Red Mage to play Blue Mage when it came out because, unlike Red Mage, Blue Mage actually succeeds at being a frontline caster (without giving up so much to do so,) it would make my long-neglected inner (melee) Red Mage very, very happy.
(Okay look, I know Red Mage melee does not work so well. I also know it SHOULD where it doesn't. Blue Mage DOES where Red mage SHOULD.)

Instead of the black magic-only Occult Acumen, I want Arcane Acumen. Go ahead and guess what that would do. (Hint: It's Occult Acumen for Blue Magic.)

Treasure Hunter.

Wide Scan. Even just one tier of it would be nice.

We have Auto-Refresh and Auto-Regen... what about Auto-Regain? How's that for giving the SAMs and DNCs a kick in the pants?

Higher tiers of everything for which higher tiers exist. Attack/Defense/Accuracy/Evasion Bonus II and III and maybe even IV but I doubt that would ever happen. It can work the same as Clear Mind or Dual Wield -- more relevant spell pairs equals higher tiers.

Triple Attack. . . . . . . What? We already have Double Attack, so why not?

Conserve TP. We already have Conserve MP, so...

More Killer effects. We don't have all of them yet, I think.

Some sort of new, unique-to-Blue-Mage trait would be cool.

Auto-Reraise. That's a trait Aern have, right? (Right, like THAT'S ever going to happen.) <<< I could see this requiring every 'crappy' spell slotted to get. And only having a (small) chance of proccing.