I don't see a city zone service doing well at all terminating at LIC/HPA, even with the growth in LIC. Most of it is residential (I heard a rumor even the Citi tower is going to be converted to residential), and I'm assuming most of the Sunnyside project will be too. Imagine if all Kew Gardens and Forest Hills trains only went to HPA/LIC, with a transfer at Woodside required for NYP/GCT. I'm willing to bet you'd see ridership drop faster than a guy with cement shoes in the East River. Running to FBA would only serve stations south of Atlantic Av, the bulk of the line is left out. Although we haven't seen an official plan, ESA slots are basically all accounted for since it seems like the goal is to redirect as many non-NYP electrics into there as possible (IE, the entirety of the Atlantic branch). I could be wrong in my assumption, but even with full time city ticket, I think you're going to see a low turn out on something that isn't direct to Manhattan. Also, any new city zone service would probably need rush hour headways of at most 20 mins (and that's really pushing it), to really draw in riders. I'd much rather see this service as a hybrid style of service, a midway between subway and LIRR, but logistically, as things stand now, I think there's more in its way than as a subway. The problem is that making it a subway can only be done one way with the current infrastructure, and that way potentially brings an issue of dangerous overcrowding.
Fantasy land would be to see the entirety of Rockaway restored to LIRR. Trains running both directions NYP to Far Rock, across the Rockaways, up the RBB to FBA, and then a second service running Rockaway Park<->NYP/GCT via the RBB. But this would require both a restructuring of fares within the confines of NYC and relocation of all the projects/section-8/housing assistance residents to someplace with cheaper rail service, or some sort of subsidization of their fare. When I talk of a build out of the regional transportation network, I'm saying something like connecting ESA with Gateway and allow through running of some shorter LIRR/NJT routes, strictly for the sake of making the most of the limited capacity. Something like, Rock Park, up the RBB, into GCT, down to NYP, then out to Newark Airport or something. It sounds wild compared to the separation we have now, but it's probably the easiest way to add capacity to our system without having to do an even more massive build out.