Here is an idea:
I'm fond of interesting, immersive GUIs. Right now we have a "default" white Roll20 interface: white for the windows, displacement bars, handouts, etc.
What if we could change the colors, or even add some texture as a "skin" or "theme". Even web browsers like Chrome support it. Virtual tabletops like Tabletop Forge (web based) or Fantasy Grounds II also allow it.
I don't dislike white color, but for instance in certain genres/settings (horror or mystery for instance) black could be more suitable. On the other hand, I know that this is an aesthetical thing, not a priority.