I always thought about this.
Quicksilver
level 2
Down + S: Time Bullet: Slows down a single target along with any enemies around them.
R1 + S: Time Lag lvl 2: Makes the world around you slower.
level 3
Up + S: Warp: Makes you twice as fast in all attacks and movements.
R1 + S: Time Lag lvl 3: Enemies stop, but DT consumption occurs even in Super Legendary Dark Knight mode.
Doppelganger
level 2
Up + S: Shadow Flyer: The shadow turns into an energy ball and is fired. It causes the enemies to be sucked into oblivion if they're weak enough.
level 3
Down + S: Phase Shift: Teleport randomly around the enemy, releasing timed slashes along with your shadow. Very flashy.