How has it gone back to 2D, exactly?
Like Sean said, there's nothing we know about Chinatown Wars, it was just announced a few days ago! Oh yeah as well as that, if you have read the news on the front page, it's back in Liberty City, meaning we can roam around Liberty City, it's just that the centre attention will be placed in Chinatown.
Yeah, we'll probably have all the contacts there.
Also going to the DS doesn't mean that it will be 2D, at least not necesarilly.
Anyway, the first GTAs were never trully 2D, some parts of it were 3D from what I heard.