While the idea of a public garden sounds great for Miami, I think a better use of space would be to create a vibrant plaza in Brickell where people can congregate for commerce. Mary Brickell Village is a bit too mall-esque to fit that bill. It would need to be something more public/private, and in the Miami tradition, why not draw inspiration from Latin American colonial cities, nearly all which are build around a central square/plaza?
San Miguel de Allende, Mexico:
Morelia, Mexico: