My idea revolves around a post sociaty world in which plants have overgrown all major cities, tearing through roads and spiraling up sky scrapers weakening underground tunnels even collapsing some in places. However this is not the decades after sociaty that first glance would have us suspect but merely weeks and though by day the world appears to be as lust and wonderous as the graden of Eden, by night the last surviving humans, maybe only a few thousand world wide, are in a constant battle to survive.
This concept is ment to be a horror RPG in which the animals of the world have evolved (or reverted to?) a mythical, more powerful and often smarter version of themselves. Idk why this happened, perhaps it could corrilate to 2012 just for the fun of it but that mystery of why this is happening now would be one of the key aspects of the game along with the constant struggle for survival that is. I see this game taking place through a day and night cycle, day being the calmer safer time to gather supplies, explore the wonderous enviornment and seek the answers to the ever nagging question "WTF!?!?" where by contrast nights are spent struggling for survival either youre tucked away somewhere safe or you are fighting for your life.
*update*
I've been working on details while I was at work, I made this up as I typed it so that's why it's a bit empty. There are defiantly sentient enemies though whether they are of the animals or the cause I'm not sure either way they are to be either demon-esque or dragoon-esque. I've been thinking the game has a natural feel for being about change so I see great potential for a storyline that does a kind of LOST thing with the characters flashing back to their life before the event and who they were and how it compares to who they have become. "Plants and animals weren't the only things to change, I use to be a garbabe man before I was a monster slayer."
The feel for the word during the day is something I emphasized on a lot in my thought process today it should be beautiful and mystical. My key inspiration for this idea was the discovery channels "Life after people" documentary, applying a mythical feel to that I see the sky scrapers entwined with massive tree like vines. The roots reach down into the subsystem beneath the city which by this point would be flooded but the massive trees obsorb so much water that they keep the flowing tunnels to a light flowing underground river. The plant all the way up sweats the water it absorbs and at the tip to floor of each tree in a spring of water that tops a then downward flowing system supporting a thriving ecosystem with the structures. Massive roots block off road ways, gas is scarce and getting harder and harder to find.
As for the sentient life, the main goal is the death of all humans so I think there is a huge horror element to making it so that the humans can not control the creatures that exist, humanity is in this alone. Something happens in December of 2012, end game probably when you would fight the boss, something to do with the alignment of the planets is why the Dragoons, or demons are doing this.
Sounds Interesting
This has my interest piqued. It sounds like it could make a fun setting, and I don't think I can recal any RPG-esc creation that was an apocalyptic world caused by the Gaia revolution, so props for creativity! I'd second what Mikeb said though in that I would like to hear more, but its off to a good start!
Has potential
I think you've got a good seed, but I'd want to hear more detail. One way that I would encourage would be to either have some of the animals turned sentient (a la Planet of the Apes) or have some humans able to control the animals through some supernatural means.