Double click, swipe left, can I change volume slider to brightness?

Short answer is No.
Until an update comes out with that in. Or if there's a JB tweak.

 

What they said. Not possible unfortunately. Would make more sense to be brightness too as can change volume with hardware buttons

 

Maybe with a jailbreak app

 

Jailbreak, and its in Cydia.

BrightnessBar, i just installed and Ill see how it works

 

Here's maybe the next best thing. This webpage allows you to create desktop shortcuts to different system settings, I use one for toggling Bluetooth and it's a big timesaver. http://iphoneza.co.za/IconSettings/ There is one there for Brightness. When you do the "add to homescreen", you'll see a prompt with the work Brightness and a temporary white icon - wait until the icon finishes loading before you press the Add button.

No jailbreak required.

 

While brightness setting would be cool, I just want a damned Wi-fi and/or Bluetooth toggle! That would be really helpful.