Allow the Shoram (nuke) plant on long island to open and the broadwater natural gas floating terminal in the sound to allow the area to be more energy independent and for the northern state parkway to be extended all the way to stony brook instead of terminating in commack. You can also have a bridge built from oyster bay to rye (been in the works for 30 years) which would reduce a lot of the traffic in upper manhattan and the bronx
These all sound really interesting. While you're at it, why not build an offshore terminal for unloading tankers (with pipeline to shore), so they don't have to nav the approaches to N. Y. Harbor, which is a recipe for oil spills...
It would be awesome if you could modernize and uncongest the airports too (maybe allowing JFK to turning into a MEGA hub for all of the airlines, saving TWA would probably help)
What about building another airport? At, say, King's Point? Or, if you'll allow an "outside" POD affecting NYC, what about adopting hi-speed rail? That'd dramatically reduce the airport congestion, wouldn't it?
If you'd accept a POD a bit farther back, what about the proposed damming of the Hudson or Harlem River?
a great POD as well is to allow the west side stadium to be built to house one of the baseball teams (preferrably the mets) AND to bring the jets into NY to play there as well. You can accellerate the nets leaving NJ and coming to brooklyn as well (the team has had attendance problems forever)
Honestly, I think the impact of this is vastly overrated...
you want a 1940s POD?
here:
Nazis get past engine glitches and successfully develop trans-Atlantic large bombers.
affecting neighborhood character, "impact" on skyline, anti-aircraft gun-towers for kids to play around on post-war, early boost for area airports with military defense requirements, take your pick
if you want to get really gruesome, throw in "V3" primitive ICBMs with various ordinance delivered on target.
maybe heavy water development gets a kind of boost.
maybe d-day goes to hell and the Nazis have extra time to develop more weapons to rain havoc on nyc.
maybe a kind of nuclear bomb with a swastika on it lands on the island of Manhattan, that would profoundly suck, but, hey, it's definitely a POD.
You don't actually need these. Recall
this, & imagine it in the vicinity of the Empire State Building. A U-boat firing into an ammo ship, or laying mines an ammo ship hits, would do nicely.
Another thought: NYC doesn't house the UN, so the Waldorf-Astoria (?) isn't torn down.
Or, if you're going back farther, the Erie Canal isn't built, which drastically reduces the growth & economic importance of NYC.
Erie Canal...15 years late, would have major changes on the character of New York. Especially the latter; if you can hold it off long enough, the B&O Railroad will be complete and we might see Baltimore a much bigger and more important city than it is OTL, sapping a lot of economic and population potential from NYC.
Baltimore? Really?

I understood Boston would be affected most. (Admittedly, working from memory.

)