Story Concept

So I've got a number of ideas just dancing around in my head, and one of them is a new MLP story. Now I know that my other stories haven't really gotten that far so starting a new story isn't really something I should get around to, but if I do, it would at least get me back into the swing of things for writing an MLP based story.
The story concept I'm thinking about has probably already been done, but here it goes;
Equestria has known peace for a thousand years, however during this 'Calm' Equestria has slipped into state of decline. It's still a beautiful nation, soldiers have lost their edge, advanced magics have been forgotten and technology hasn't really advanced in the past thousand years.
However with Nightmare Moon's return it issues the end of the Calm, so old and new villains not to mention other threats like monsters are growing more frequent by the day, and because Equestria hasn't been preparing for the Calm's end, they are caught off guard and struggling to deal with the problems brought to their doorstep. So they have to spend their time getting the guard in order and retrained. Meanwhile with news of what's going on, adventurers are now coming to Equestria to plant their hooves into the Equestrian soil, fight monsters and explore the lost depths of old ruins.
With this setting, I want to have it OC centered with the cast just going around doing adventures, forging bonds and overcoming personal trials, at the same time I'd probably have a rotating party for a while. Meanwhile Celestia would send a young and possibly more anti social Twilight Sparkle out to become an adventurer so that she might search for the Elements of Harmony, which are either hidden away in different locations or in Nightmare Moon's possession.
Finding the elements would be Twilights personal mission, while everyone else's would just be to have fun and enjoy the adventure, but they either get roped into helping Twilight or just stumble into her quest anyway so Nightmare Moon might be defeated. But, even if they succeed, that doesn't mean that the world suddenly gets better; the Calm is over, more monsters are appearing and where one evil falls, another simply rises to replace it.
And in the background of it all, you have Discord just waiting in the winds.
Anyway that's the idea I got for a story, feel free to tell me what you think in the comments