I've been a Dual Blade since 2012 and the class has a special place in my heart.
A nice improvement would be changing the sound effect of blade fury.
Holy shit man that thing is loud and probably the #1 reason why I keep everything muted now. Just put on Blade Ascension's sound effect on it or something because if you kill like 2 or more mobs it's deafening.
Another bigger and still realistic change would be the ability to climb ladders while using Asura's Anger. Right now you can jump/flash jump during it which is nice but if you fall off a platform when you're doing horntail or killing mobs you either have to sit and let the skill time out or manually stop it in order to re-position.

Final Cut also needs its range increased to where it was before.
Why did nexon even shorten the range on it (was it gollux???)
Dual Blade's are already one of the lowest ranged classes in game sitting next to like white knights or something, they shouldn't be punished further and increasing the range of FC wouldn't be gamebreaking because right now you need to practically be in Phantom Blow range to proc final cut.
An unrealistic change would be scrapping Blade Fury and giving them a larger mobbing skill. Seriously with all the changes and buffs to other classes Dual Blade's still feel subpar even after the range increases on Phantom Blow and Blade Fury.
There's no reason why someone would pick Dual Blades over a Shadower or Nightlord/Nightwalker unless they are a diehard fan. Dual Blades are harder to fund, weaker and have less smaller hitboxes. Shadow meld is the flagship passive of Dual Blades and 100% crit chance is nice but the tradeoff when comparing the class to a ranged NL/NW is tricky to judge.

I think the most glaring problem of this game as a whole is the fact that almost all buffs last 180 seconds. If you have any say in overall game balance this would be the game changer. When classes have 6+ buffs they need to use it's pretty stupid that every three minutes they have to stand still for 10 seconds because buff animations take so long.
In upcoming patches classes including dual blades received shorter buff animations but even then the durations are still 180 or 200 seconds. Buffs should last 6 minutes or longer.
I'm level 203 now and hope Nexon give our class some love