Every plane in the multiverse has something to make it notable - Lorwyn/Shadowmoor has days and nights that last for years, Mirrodin is a world where metal infuses every form of life, and Kamigawa is a Japanese-inspired plane where gods and spirits can cross over into the physical realm. Ravnica's hallmark is simple: The entire world is covered in one vast city. Nature's been pushed aside in favor of the dominion of the ten guilds. Only the Gruul and, to a degree, the Selesnya, have effectively resisted the encroach of civilization. But some cards in Return to Ravnica and Gatecrash may foreshadow a naturalist revolution in the final set of this block.
Return to Ravnica's Wild Beastmaster is a guildless Shaman, and her message comes through loud and clear in her flavor text: "When they covered the world in city, all they did was give themselves nowhere to run." This indicates that the green elements of the Gateless movement may have to do with taking the guilds to task for their suppression of the course of nature.
From the same set, Urban Burgeoning indicates that even on Ravnica, nature can swiftly move to reclaim abandoned areas of the city: The ruins of Old Prahv became a wildspace, although all birdsong was silenced by lingering Azorius magic. Perhaps we'll see more of these "wildspaces" in Dragon's Maze as parts of the city are desolated by the guilds' race to uncover the Implicit Maze.
Gatecrash's Ripscale Predator clearly shows nature gaining a foothold on the city-plane: As Ravnica's wilds expanded, even advocates for nature had to confront fiercer natural threats.
Finally, thanks to the presence of Merfolk, we now know that Ravnica is founded upon a single vast underground ocean. This is unsettling information for the inhabitants of a city, to know that they dwell not upon solid ground but an endless abyss of water. Might Ravnica sink as the final act in its story? We'll find out once Dragon's Maze is released.