Just an opinion, don't go shooting arrows at me (just kidding of course) but:
I personally never really considered a Nassau Street Line running via 4th Avenue again because before 2010, ridership along West End and 4th Avenue was very minimal. However considering how most Brooklyn riders transfer to the & IRT lines between DeKalb Avenue and 59th Street, it sort of seems like the most practical option.
Possible Option:
: Jamaica Center <> Bay Ridge-95 St via Jamaica Skip-Stop Express/Nassau Street-4th Avenue Local, Rush Hours. Other times, Jamaica Center <> Bay Ridge-95th Street via Jamaica-Nassau Street-4th Avenue Local.
Pros:
Due to minimal interlining (only interlining with the at Essex Street and Myrtle Avenue-Broadway versus the interlining with the at Queens Plaza, at the 60th Street Tube, at 34th Street-Herald Square, at Whitehall Street and more recently at 36th Street after 7PM due to Work Train assembly and at 59th & 36th Streets due to construction that causes the to stop at 45th & 53rd Street) delays and horrendous service gaps of 15 minutes+ would be reduced.
Direct South Brooklyn & Nassau Street service would be restored.
Direct South Brooklyn <> North Brooklyn service (believe it or not, I know many people including myself that would highly benefit from this).
Nassau Street and Lower Manhattan Broadway stations are very close in proximity, minimal impact to Lower Manhattan riders.
Direct access to major hubs such as the Fulton Street Transit Center, City Hall & more Chinatown access.
If the few Brooklyn Heights/DUMBO riders really rely on Broadway Service, a transfer would be available at DeKalb Avenue or Canal Street to Broadway Services.
Bay Ridge and 4th Avenue riders would finally ride on newer upgraded fleet, a step up from, the dingy R46's.
Cons:
Correct me if I am wrong but I believe as a terminal, Bay Ridge-95th Street can only turn around so many trains an hour (I think the number is around 10). The combined run a total 12 trains an hour (a frequency of 5 minutes). If 10 trains per hour is the maximum capacity that Bay Ridge-95th Street can turn around, 2 and/or trips would have to be short turned at Broad Street, causing a slight gap in Rush Hour Service in Southern Brooklyn.
If the short turn option is a no-go, then the would have to be reduced to a combined 10 trains per hour (a frequency of 6 minutes) which doesn't seem bad at first glance, but riders along the Jamaica Line would experience 12 minute frequencies (average 6 minute wait times) at Skip-Stop Stations. I don't think Jamaica riders would be too enthusiastic about that. However I believe it can be done because if you recall the was in fact going to be reduced to 10 trains per hour as apart of the 14 Street Tube Shutdown Plan to make room for beefed up service.
The would deal with more interlining due to all temporary construction along 4th Avenue, may cause delays to ripple along the entire Jamaica-Nassau Line.
May ironically turn into another long Local Queens through Brooklyn line where a delay in Queens may cause delays to ripple all the way to Bay Ridge, thus solving nothing.
: Forest Hills-71 Avenue <> Whitehall Street, all times except Late Nights (see ).
Pros:
Less Interlining = more reliable service.
Short, Sweet and to the point Local line connecting Queens and Manhattan.
Cons:
May have to short turn trips at Canal Street during Peak-Hours if Whitehall can't handle the 7.5 trains per hour (frequency of 8 minutes) that the currently runs during Rush Hours. (May not be an issue considering the runs a single train per hour less.)
Reduced service south of Canal Street (back to post-2010 through pre-2016 frequencies) with only the serving lower Manhattan.
: Astoria-Ditmars Blvd <> Coney Island-Stillwell Avenue via Broadway Local/4th Avenue Express, all times except Late Nights. Late Nights, Astoria-Ditmars Blvd <> Coney Island-Stillwell Avenue via Whitehall Street to compensate for loss of late night service south of Canal Street (unless Late Night Queens through Manhattan service could be considered). Peak Service would have to be beefed up from 7.5 trains per hour (frequency of 8 minutes) to about 12 trains per hour (frequency of 6 minutes) to compensate for the loss of the .
Pros:
Extra service along the route, especially for Sea Beach Riders.
Cons:
May need to Short-Turn trains at Kings Highway or 86th Street-Gravesend during Peak Hours (not a huge deal in my opinion) due to beefed up service.
Beefed headways can cause more congestion on the Manhattan Bridge and Merge from the Express to the Local tracks along Broadway, delaying service.
Absorption of the would upset the many lovers of the line (including I).
: Retired, much to my dismay, but if the MTA comply's with Bay Ridge's demands, I feel there is just no room for the in this scenario.
In conclusion, there is no perfect way to solve the unpredictable service along the 4th Avenue Line. Chances are, the best service pattern for the aforementioned riders... is the one we currently have now!
It remains a very damned if you do, damned if you don't scenario. It is very possible that there is no perfect solution with the way the subway system was built over a century ago. There just isn't enough time or money to reconfigure the subway system to meet 2019's demands.
Thats just my 2 cents, I don't feel that the is going to rearrange service, it is probably fine left just the way it is, even though it is pretty crappy. It's the best that can be done.