I think it would be very cool if it was based off of New York! But honestly it doesn't make much sense to me that it would be.
My brain is telling me it is based upon Shanghai not just for the reasons stated in the video posted earlier in the thread, but also because it is directly West of the island of Kyuushu, the island that Hoenn is based upon. And whether or not Hoenn is able to be visited in Black and White, there seems to be a great amount of expectation that Ruby Sapphire and Emerald should be remade in this, the fifth generation. The connection geographically alone seems to hold it in my opinion, no matter how much I would think it would be cool if it was NYC, Shanghai is also a large port city, and the entire region surrounding it when compared to the Isshu region map is approximately the same size as past pokemon regions explored, where NYC is much much smaller in comparison. Traditionally Pokemon games are not exact copies of their base regions, this is true, but they usually follow them closely enough geographically for Shanghai to be a much more appropriate area for the Isshu region to have been based on.
Pokemon games typically follow asian culture fairly closely, so that puts China in a better position culture-wise to be selected as a foreign region rather than the United States. There have been numerous references to Chinese culture throughout the history of Pokemon, the most notable being Growlith and Arcanine, two Pokemon who are originally native to China. The majority of the new Pokemon seem to exhibit the mashing of East and West that Shanghai itself exhibits as well, with the gothic inspired pokemon being an example of the West and the daruma doll inspired pokemon being an example of the East.
Even the desert juxtaposed right next to Hiun city speaks of Shanghai's own environmental issues, in the way that its rapid growth quickly led to the degradation of the local environment there. In modern times, the most extreme example of this is desertification, where all the water is diverted from one area so that another may flourish and expand, causing the death of the other region. The "Hiun" in Hiun City even means in japanese "bad fortune" or "cloud flying through the sky".
Isshu can mean "one complete year", referring to the four seasons that Isshu experiences, and the Shanghai region is a perfect candidate for this, as it experiences 4 distinct seasons despite being as far south as it is.
Shippou City is named after the 7 treasures of bhuddism, an eastern concept, and can also refer to Cloisonne, originally a chinese art style. Karakusa town is named after a type of artwork from the middle east used in Islam (is it a coincidence that Team Plasma is first encountered here, or am I alone in thinking they had a very crusader-era look look going on?).
Not really looking to argue any of this, was just throwing it out here as food for thought +shrugs+