Ok, for something to be super-huge the defection can come from only a few developers, those that own multi-million selling franchises, the total cream of the crop.
We have:
EA - these can be excluded since they're multiplatform and thus no defection is possible
Rockstar - GTA while releasing on PS2 first also gets released on the PC and Xbox so I don't know how much of a defection this would be.
Nintendo - not gonna happen
Bungie - not gonna happen
That just leaves
Square-Enix - Other than Nintendo, no other Japanese company pulls in the numbers that SE does. Add this to the fact that they're completely 3rd party with no obligation towards Sony. Plus, this would address Xbox's biggest weakness: Japan. So if it's really going to be something industry shaking, then the only choice would be SE to MS. Nintendo already does pretty well in Japan so SE going to Nintendo wouldn't have the same impact as if they went to MS.
Oh, and if you want to get really nitpicky about Weapons, that's exactly what every final fantasy has as their uber-boss.
Even Namco, Capcom and Konami all going exclusive to Nintendo, or MS wouldn't have the same impact that SE would.