I dont agree with neither shanghai nor NY theory, but i think it will be based on a place in japan like every time, they said that isshu is far away from other regions so it would match the story and the fact that there are only new pokemon there, but real world and pokemon world are probably different.
From what I see most of ppl here think it is based of NY, but I also noticed that most those ppl had stupid reason for believing in that, like:
"If there are a lot of pokemon with a red,white, or blue coloring, it's probably America"
- -.- yah, riiight
"3 on 3 battles... reminds me of 3 on 3 basketball. I would think basketball is big in NYC so maybe that's a reference?"
- I dont see how that has anything to do with 3on3 battle
"The older main characters are based on that Americans are much more protective over their children (this is true.)"
- This one was just funny
"The Subway....okay what's one of the most infamous things about NYC? The gross subways."
- Subways are more like Japan, NYC is not infamous bout that at all thats just dumb
"The trailer suggests that there are backpackers/homeless people"
- And?
"People of varying nationalities"
- This one is stupid too
"Docks"
- Oook :S didnt knw other countries have docks
"Protagonists clothes"
- Wtf?
And the most popular, geographical location... when u look at the picture on isshu and NYC, the only similarity are those 2 rivers, beside that its not similar at all, I mean check the other parts, totaly different, same goes for Shanghai.
As for the fact that you can only get to isshu form other regions by plane of boat...well Japan is an ISLAND so you cant get anywhere else except by boat or plane, so it could be everywhere.
Maybeh im wrong, but I highly doubt that gamefreak would put a region in America.