I think it would be awesome if instead of being just limited to tinted colors for auras, a token could have another image as its aura which would scale depending on the aura's size. Images such as those found in Environmental Effects 2 would be a prime example of the sort of images used for aura image tokens. I dont know how difficult it would be to accomplish, but it is something that I personally would like to see and have a hard time seeing it as particularly difficult to implement.