Hi! I didn't know about this forum until just moments ago when I read about it in the "New York Times" -- congratulations on the coverage!
I'd like to share my enthusiasm for a fantasy route that has always interested me -- in the hope that maybe some others with more knowledge and expertise will also find it interesting and consider it for their own future fantasy maps. (Jayman, I hope I have not broken any rules of etiquette by posting this in your thread.)
One thing I've noticed, is that whenever transit enthusiasts talk about additional transit lines, they always seem to think in terms of what might be called, "more of the same" -- more lines into and out of Manhattan, or more spokes on the wheel, so to speak.
However, as a lifelong New Yorker who was originally from the "outer boroughs," one thing that has always bugged me about NYC mass transit is the fact that you basically have to travel into Manhattan if you want to go from one borough to another. (Practially speaking, I believe this is one of the reasons that various outer borough "hubs" have never really taken off the way they might have if they hadn't joined NYC in 1898.)
So I hope people will enjoy hearing about (and consider fleshing out) the idea of a fantasy transit "beltway" that would connect the various outer borough "hubs" (and "hubs" to be). I'm thinking of a double track (each line in the opposite direction) monorail line that would be light enough to travel over bridges and also allow you to transfer to existing spokes in the wheel (the way the No. 7 allows you to transfer to the "E" or "F" train). (Perhaps in some areas of the city where there are already "els," an underground monorail would be a better interface.)
Roughly speaking, here is how the route would go.
1) Across the Bronx (above or beside the Cross Bronx Expressway?) and over the Whitestone Bridge;
2) Across Queens (crossing over and connecting to the No. 7 and the Hillside Ave. IND line) and then over to Brooklyn.
3) Across Brooklyn (crossing over the various lines radiating out from downtown Brooklyn) and then across the Verrazano-Narrows Bridge
4) Across Staten Island (crossing over the SIRT) and across the Goethals Bridge
5) Then northward through Hudson County and back over the George Washington Bridge.
As part of this fantasy transit "beltway," the No. 7 would go under the Hudson and link up the New Jersey portion of the beltway. So this line with link up with the Beltway at two points: Flushing and somewhere in New Jersey.
I realize due to political and economic considerations, that this is really a wildly impractical idea. But since people are talking about fantasies -- why not think out of the box?
What I like about this fantasy is that all of a sudden it makes the spokes going into and out of Manhattan less of a "one-way" street (e.g., into Manahttan during the morning rush) and more of a two-way street (into and out of Manhattan during the morning rush). With a transit beltway, a business in, say downtown Jamaica, would better be able to attract customers and employees from the Bronx, Brooklyn and Staten Island. So people in Manhattan would then too have a reason to take the subway out to Jamaica during the morning rush hour -- making the entire subway system better utilized. (Instead of all the users traveling one way into Manhattan, you have users more evenly distributed -- giving you, in effect, two transit systems for the price of one.)
(P.S. -- Many of you may already know about this, but there is a great monorail enthusiast site that addresses many of what it feels are the myths about monorails. I'm suggesting monorails in this fantasy because they appear to be lighter and cheaper to build. Plus they don't seem to make the noise that elevated trains do -- my biggest complaint about the "el" that used to run along Jamaica Ave. when I was a kid.)
Unfortunately, that was my sentiment exactly. 
