Allowing to change the z-index for bars would help with overlay items. For example if XP / reputation bar is enabled, they overlay the Blizzard Art while in the default version they are drawn behind the art.

A simpler solution would just be to draw the Blizzard Art over the XP and reputation bars by default.

Default

Bartender

As a temporary workaround you could write a little Addon yourself that loads after Bartender and sets the https://wow.gamepedia.com/FrameStrata accordingly.