Making kratos a permanent part of MK is an awful, awful idea, sounds like something a PS3 fanboy would come up with before thinking it through (and I say this as a PS3 owner). He already has his own game, hes in God of War, leave him in God of War, don't start trying to work him into MK as a proper character, its just a recipe for disaster and compromises the integrity of MK and its story. I can't see NRS letting that happen after they've just rebooted the franchise and put so much work into building such a good story for it.
And theres absolutely no way MK would go platform exclusive, the days of platform exclusivity for thirdy party games are essentially over. The closest you get to platform exclusivity these days are different bonuses depending which platform you buy the game on (such as Kratos). Gone are the days when Sony had a stranglehold with the PS2 and could make exclusivity seem like a good deal, 360 is selling more and has a bigger market share than PS3 even these days so no third party developer in their right mind is going to exclude either platform. Remember when GTA was a Playstation exclusive? And then with GTA4 it went multiplatform and actually went with 360 as its lead platform? The times they are a changing.