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Realistically though I think Nintendo is so ignored these days that they probably could have an incredibly original and awesome system-selling idea right out in the open and no one would steal it until after the game became a success.
Well, there is always Jason Rubin, of Crash Bandicoot/Jak and Daxter fame.
He's pretty much admitted in numerous interviews that he steals every idea that anyone else in the industry has (especially Miyamoto, who he "respects deeply"), and once he does he's suddenly better than everyone else in the industry, because he's got all their ideas,
plus his own ideas.
But really, what are you going to do to stop someone like him? Refuse to talk about a game six months before it's release, and just release it quietly on launch day, and the most you can hope to accomplish is delaying the knock-off's release by about six months. Unless the silence kills your game, and nobody thinks the ideas in it are worth stealing.
I think it's just "Nintendo's way" to be ultra-secretive, and it's a personality thing, not calculated.