The likely choices for the next main entry for the KH series (and why) would be these:
PS3 Exclusive - Nomura loves the PS3, Sony is the very reason Nomura has a career, so from a director standpoint Nomura definitely wants it to be on the PS3. Nothing more really needs to be said, as I don't need to go convincing anyone that KH has a chance to be on the PS3.
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PS3/360 Multi-Platform - Let's face it, there's no way the 360 would get it exclusively, that would just be a bad business move, but the chance of Square Enix putting the game on the 360 in addition to the PS3 has a chance, as Square Enix said they would like to support all consoles evenly. So a Multi-Platform release would be supporting the 360 as well as the PS3, and honoring Square's vow to support everyone with even distribution of their hit titles, not to mention getting the game to a wider audience, and the fact that KH does very well in America, which is the 360's strongest region
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Wii Exclusive - The Wii sells exponentially to the exact demographics that Disney and Square-Enix target the KH Franchise for, so it's an extremely safe bet that the majority of Wii owners are the kinds of people who would love and buy up the KH series. Plus the fact that the Wii is the highest selling next-gen console on the planet, in addition to the benefits of the console reaching the same demographic as the series, there are many more Wii owners than there are PS3/360 owners, which means as a financial decision this is even more potential money for Square Enix.
Kingdom Hearts is not really a graphically intensive series, so the Wii gives enough extra power for Square-Enix to give the series a nice boost upwards (just not super-HD), and giving the title to the Wii would be showing Square's faith in spreading out their big franchises to all 3 consoles
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Reasons any of these respective options may not be likely
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PS3 exclusive - As said before, in 2006 Square Enix stated that they no longer wished to support Sony as exclusively as they had in the past (not over-support the console, but not under-support it either), as always keeping to one platform really limits the demographics and audiences they could reach for their games, so Square Enix stated they wanted to support all Next-Gen consoles and all Handhelds with an even distribution of their hit titles.
So, as the PS3 has Final Fantasy XIII and VsXIII, Square has already covered the bases with the PS3 in giving the console a huge hit title, giving the PS3 the next huge entry in one of their other huge franchises would be over-support for the PS3, and under-support for the other Next-Gen consoles, which is what Square-Enix said they wanted to avoid. So with Dragon Quest IX on the DS, Crisis Core and Dissidia hitting the PSP, the 360 and the Wii are ample consoles left for Square to give their support.
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PS3/360 Multi-Platform - Well, there's not much here to say that this is necessarily a terrible decision, Multi-Platform releases are pretty much safe-bets as the game is simply reaching more people on more consoles. Well, the main detriments I can see would be splitting the fanbase up, or that perhaps the 360 audience are not the demographic that would buy a KH game and may simply result in the game having lax sales on the 360, which would end up with Square's advertisement money for the game going to waste.
You could suggest that perhaps since it's multi-platform the game may have sacrificed quality in order to properly function on 2 consoles, instead of being developed exclusively and tailored specifically to one, but imo Square Enix is really not a company that skimps off on something like this, if this is Square we're talking about here they're gonna blow us away with whatever they do, so having the game on 2 consoles isn't something they're going to allow to hurt the overall game.
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Wii Exclusive - The Wii is an unknown element for Square at the moment, they haven't ventured from Sony in quite some time, so it's probably an awkward situation for Square here as they haven't tested the Nintendo console waters in over a decade. They may be unsure of how well the game can perform on a console that really tries and sets itself apart from the average console, and thus has a completely new and untested audience buying the console that Square really doesn't know what to make of.
Square's newest effort on the Wii is the Wiiware title "Final Fantasy Crystal Chronicles: My Life as a King", which may be Square-Enix's testing of the Nintendo waters to see if a game of theirs can perform well on the console, so depending on how well that game sells on the Wii may be the deciding factor on whether Square will commit to Nintendo or not.
There is also a similar situation to the PS3/360 scenario wherein the Wii's unique controls being a new element for the series, it may be something that could go either way for the game, either make it completely awesome and a new experience for the series, or end up really awkward and you wish you could just play with a normal controller again. My feelings, like previously, is that if this is Square-Enix we're talking here, Square Enix isn't going to give us crappy controls, Square-Enix is going to work their ass off to do with the controls what no one else is bothering to do, and Square Enix would be the one to really work at the controls and make it great for everyone playing.