Idea for 5th Ave Subway:
Jamaica-Parsons/Archer to 59st-5th Ave, Manhattan
Bay Ridge-95st to Jamaica-179st
The 5th Ave subway starts at 1st Ave and Delancey Sts where 2 tracks from the Nassau Line and the 2 from the Williamsburg Bridge form a 4 track trunk running up 1st Ave. The line then turns down 14st (Under the ) with a stop at Union Square. Then the line heads up 5th Ave making these stops:
23rd st
34th st
42nd st
50th st
59th st
Two tracks terminate at 59th st (provisions for extension built in) for the while the other two take over the 63rd st tunnel to Queens.
Queens Blvd would look like this:
Current service pattern
71st-Continential to CI via QB Local
179st-Jamaica to Bay Ridge via QB Express/63st tunnel/5th Ave line
Same except cut back to Whitehall.
The reason I think this works better than continuing the 2nd Ave line south is because a line down 5th puts you within walking distance of all the offices along Park/Madison/6th aves. People will use this versus trying to pack the Lex.