Actually i have gone beyond the business plan stage...
I mean with your idea you haven't got the funds to fully develop and mass-market it. So with the aim of developing further in versions/sequels, you release a watered-down version (so the idea is out), which means the rich guys (big companies) with the huge marketing resources see the idea and your toil becomes their gain
So don't make a watered-down version of your game if you're that worried about it. I think this is actually not as much a threat as you make it out to be, but in any case trying to make a big splash against the AAAs by playing their own game is very much a case of your reach exceeding your grasp -- unless you can get someone to give you enough money to play that game effectively.
But you're falling into a false dichotomy, because you don't have to go big-budget or go low-budget. You could make a different game, and thereby build experience and perhaps earn a nestegg with which to take on a bigger idea.
These days cool ideas comes about ( in addition to intelligence and creativity) by: accidentally, experimentally or both, otherwise it will be obvious and would have been done.
If you don't agree you are saying the rest of the world is not as smart (or maybe dumb) - in other words you are saying "i can easily see what other cannot see"
Think of all the thoughts you've had and never executed on -- and not just executed because it was a bad idea, but not executed because life stands in your way, or because watching your favorite television show is just too important to you to give up in persuit of an idea. You have more ideas, I'm sure, than you would ever get to in 10 lifetimes. Everyone is like this. Follow-through is far more important than an idea, and its far rarer. Many good ideas will have been thought of already (at least in broad strokes), and some ideas do have a kind of latent value, but no idea has extant value until its made real and shared.