Yeah, not sure how to get the city proper to the number one spot, but maybe if crime in NYC remains bad post-1990 and if crime has an NYC level recovery in the D.C. area, the Washington metro area COULD be the number 1 metro area years from NOW.
For an extreme example using a recent POD, how bad can you get the crime in New York to be in the 80's? How bad does it have to be to getthe art scene to at least partially leave the city? Or is that just ASB? Probably is. Not that those people made up a lot of the population, but when city living became cool again, NYC being at the center of arts and culture put it at the top of young people's lists. If you can somehow get the yuppies, artists, hipsters, overseas nouveau riche to move somewhere else starting in the late 80's, you might be able to build momentum for later this century. But not by the present-day. New York just has too big of a head start. You're gonna need an early POD to pull this off.
My instinct would be to get the center of immigration shifted to a different city somehow, but while that might kick New York off the top it's not going to help D.C. as said other port city will probably be Number 1 instead.