That is an excellent suggestion. I wish I know who you were I would reward you for thinking that way. I actually think it has been tried in Mexico city on a few buildings.
As cities continue to expand we are gradually replacing the forest cover with concrete buildings. It would be an excellent suggestion to have the tops of buildings have some form of plants. Obviously it cannot be trees due to the damage their roots could do but I would support planning smaller shrubs and bushes. Great idea future scientist!!