from a realistic point of view:
Potential Candidates that can be removed:
(Q) - the N train serves all of it's stations, basically
(Z) - the J train serves all of it's stations, basically
(D) - shares same terminal as the B in the Bronx and Manhattan. the G, if trackage permits, can take over the D in brooklyn, as it will not be possible through the B.
Potential candidates that CAN face a reroute:
(A) - can be permanently assigned a single terminal, either Lefferts blvd, Rockaway Park, or Far Rockaway, rather than 3
(B) - should the current Q be eliminated, the B can terminate in Coney Island.
(C) - can be extended from Euclid ave, to either Lefferts blvd, Rockaway Park, or Far Rockaway. Late nights, it can run as a shuttle from Euclid Ave, instead of it's current Manhattan Terminal.
(4) and (5) - can permanently terminate at Bowling Green, as a cost saving measure, rather than Brooklyn, with the (2) and (3) retaining these Brooklyn terminals.
Rockaway (S) - should the A or C terminate in Far Rockaway (the other terminating in Lefferts Blvd), it can run as a full-time shuttle. it can also be re-lettered as it's fomer H letter, or as a K, respectively.