Actually, steampunk isn't always set in the past. There are quite a few post-apocalyptic steampunk books (Mortal Engines is the first series I can think of) taking place the future. It's the future, but a steammpunk future.
Cool! Then I guess the next question I would have is, what is the backstory of your world that is leading towards that future? Is it post-apocalyptic? What sort of apocalypse?
If current transportation technologies are knocked offline, and Manhattan becomes, still a great city at the center of world commerce, but with more expensive energy thus more movement by foot, streetcar, ship, and dirigible. The conditions of the Five Points were at a time of mass migration into the cities from the countryside, and a second Industrial Revolution after the cataclysm could produce a similar situation.
Here's one vision of the future. I think it's a bit overwrought, and not critical enough of the New Urbanists (Kunstler is one), but it's a powerful image.
http://www.kunstler.com/mags_cities_of_the_future.html
I think this will lead to an epochal demographic shift, a reversal of the 200-year-long trend of people moving from the farms and rural places to the big cities. Instead, I believe we will see is a substantial contraction of our cities at the same time that they densify at their cores and along their waterfronts. A preview of this can be seen in Baltimore today. The remaining viable fabric of the pre-automobile city is relatively tiny and concentrated in the old center around a complex harbor system. With little need for industrial workers, vast neighborhoods of row housing built for them are either abandoned or inhabited now only by such economically distressed people that abandonment is inevitable. The pattern of contraction may not be identical in all American cities.
This situation is depicted masterfully in
The Wire, which takes an unsentimental (but realistic, as the guys on probation I know can attest) view of these contracting neighborhoods, which are the Five Points of our time. The Five Points, however, was at a time when the city core was growing, not contracting. This could explain why Lower Manhattan is a slum once again--it is one of those neighborhoods, like West Baltimore, that was largely abandoned after gentrification--the condo high-rises became ruins, the government facilities were relocated to smaller quarters, old vacants were razed to make farming plots, and then, when the steam-powered factories went up, people flooded back. This would have to be quite a cataclysm to depopulate lower Manhattan, but if it happened, sooner or later the people would come back, and the Five Points can be a dense, low-rise, five-street corner for the unwashed masses once again.