Like the Masters of the Universe film I'd have them coming to modern day America so that people can relate to it.
I'd replace pikachu with a talking dog that I'd call Dogmon, which would be played by Warwick Davis in a suit and dog mask. Dogmon would be the only 'pokemon' in the film and there would be no pokemon battles because this might upset Peta and American moms in general.
Ash would be played by Justin Bieber so that the movie would appeal to the teenage demographic.
None of Ash's companions from the anime would be in the film, but when he turns up in America he could be befriended by a cool black kid from the ghetto who works in a youth centre and is always saying things like "winners don't do drugs" as this will discourage the teen audience from taking drugs. He will probably be played by one of the black ones out of TLS.
The bad guy who they have to defeat can be some Russian Communist who wants to implement communism in America. As America hates fairness in society this is a ready made bad guy for Americans to root against and will attract every patriotic American to see the film just so they can see him getting bitten on the bum by Dogmon and renouncing communism as one of the greatest evils ever created in the final scene.
The trailer will look something like this:
http://www.youtube.com/watch?v=-XSq0PvVxMs 