If I were mayor, there would be a whole neigbhorhood devoted to street art. The Graffiti District. Every wall in the district would be legal to paint on. If you owned property in the district, you would be required to provide your exterior walls as a canvas for artists.
Of course, not everyone would have to live in Graftown. If you prefer blank walls, you could choose to live in one of the many other neighborhoods where the usual no-graffiti laws apply. But citywide, all municipal surfaces (subway cars, concrete freeway overpasses, etc.) would be legal for graffiti. The law would state: any public property whose surface doesn’t serve some other purpose must also be made available as a canvas for citizens to make art with.
Corporate advertisers would be subject to the same laws as graffiti writers. But because advertisers make a profit from their use of public space, they would be charged a space-rental fee.
Graffiti writers caught painting illegally on other people’s property, or covering important things like street signs, would be required to do community service: teaching style and technique to school kids (if they’re good) or buffing their own spots (if they’re a toy).