The second stage of the popular new elevated park, running from 20th to 30th Streets, opens next spring. The design will include “a ‘flyover’ where the walkway rises above the High Line’s level, and into the shady canopy of sumac trees” and “a dense stretch of trees and shrubs” described as a “version of Central Park’s wild and woolly Ramble” and named the “Chelsea Thicket.” (Ahem.)