No NX service can be added in the next 2-3 years because the express track is being used in regulars service. It is now being used daily since the northbound local track is out of commission to rebuild the stations. Once the northbound local track is back in service, the southbound local track will need to be taken out of service to rebuild the southbound platforms. After that work is complete, the temporary platforms at 8th Avenue and Bay Parkway can be removed and the express track will not be used in any regular service. Only local service will operate, as in years past, using the outer tracks.
It makes little sense to rebuild the other Sea Beach express track to do express service to Coney Island on summer weekends. They can just use the West End Line express track, but with no stops as the southbound track and the Sea Beach express track as the northbound track, or use the Brighton Express tracks, with or without stops for express Coney Island service. The B-Train runs weekdays only to Brighton Beach, so maybe it should just run weekends in the summer. On weekends when it runs, it should be able to be extended to Coney Island using the Q tracks since terminal space isn't as much of an issue on weekends.