Thinking about it, I can't see any complications in having a livery editor for the standard cars. When you consider you can change the body colour on most of the cars in the game, it shows that the space/dimensions in which you can place stickers has already been set, excluding the windows and lights that is.
I do think, however that in order for PD to develop a successful livery editor, they need to ensure that all the cars are up for the task. The current low res standard cars just wont do. Place a sticker on the body panel of a standard car, and it would appear really low res or bumpy, for the surface would be unpolished.
I've mentioned this time and time again, PD don't need to convert all the standards to premium for GT7. That would be way too time consuming. They can, however upgrade the exteriors, so all the cars are semi-premium.
In GT7, I want to take a Subaru Impreza Rally Car from 1999, remove its livery, and create my own personalized one, with tributes to the late, great McRae and Burns, and references to my favourite Youtubers and general media.
I pray thy PD, that you give us this unique ability! For all cars!