Please keep all information regarding this upcoming shutdown in this thread.
Superstorm Sandy
In the aftermath of Superstorm Sandy in 2012, vital components and infrastructure was severely damaged when the tunnels were inundated with over 7 million gallons of salt water. As a result, the 14th Street tunnels were among the last tunnels to open following the gradual restoration of subway service in November 2012. While service continues mostly as normal, the stopgap measures used to allow 
trains to run normally, the severity of the damage incurred will require a more thorough approach and another long-term closure, much as was required with the Montague St tunnels a few years ago.
The Options
Ever since the 14th Street tunnels were reopened and it was determined more work was required to get the tunnels back into a state of good repair, the MTA mulled over two options in regards to the tunnel closure. The first option was a full closure of the tunnels and the Canarsie line in Manhattan. The second was a partial shutdown, where one tunnel would remain in service throughout the duration of the repairs. This option would allow for limited service between Manhattan and Brooklyn. However, the cost of having one tunnel open would be that the overall project would take twice as long. Under option 1, the repairs are slated to take around 18 months to completion, as opposed to the three year duration under option 2.
After a lengthy process that involved more community outreach than is usually seen from the MTA, the agency decided to go with the 18-month option. While neither option was ideal for riders in the affected communities in Williamsburg and along the 14th Street corridor, the majority of those asked felt that a shorter construction period outweighed the benefits of limited service between 8 Avenue and Brooklyn. That means, starting in 2019 and lasting for approximately 18 months, there will be no train service between Lorimer St and 8 Avenue.
Alternatives
The 14th Street tunnel shutdown affects many riders not just in the aforementioned areas, but various others as well because of the line's location. As such, riders will need alternatives to get around. In the past few months, the MTA released some details on how service will be modified throughout the tunnel closure. Per usual when train service is suspended between Brooklyn and Manhattan, 
train service is extended to the 6th Avenue line during off-hours. One tentative plan from the MTA calls for 24/7 service between Manhattan and Metropolitan Av. While the specifics are currently unavailable regarding weekend terminals, it's believed the line will likely run to 71 Avenue, as it does on weekdays, or to the 96 Street station on the 
line should that prove to be unavailable.
Along with that expansion of service, the agency plans to add more cars to all 
trains, making them full-length in anticipation of the riders transferring to/from the at Lorimer St-Metropolitan Av. To facilitate transfers between the Canarsie line shuttle buses and the Jamaica line, peak-direction express service between Marcy Av and Myrtle Av will be suspended. 
and 
trains will run local at all times in order to prevent overcrowding on trains. On the subject of transfers, two out-of-system transfers will be implemented: between the and 
at Livonia Av and Junius St, and between and at Broadway and Hewes St.
As these measures alone will likely not be enough for the many that make up the line's ridership, the MTA is working with the DOT to adjust ferry service to accommodate additional riders. A new ferry route has been proposed to provide a direct link between Williamsburg and Kips Bay. On the Manhattan side, the M14 bus line will be converted to a Select Bus route to expedite off- and onboarding. There is also talk of converting 14th Street into a bus-only street.
As more details emerge, this thread will be updated with the latest information. While I will not limit what is posted here, this thread will be moderated with more scrutiny than usual. Please do not flood this thread with your proposals of what should run wherever. Once is plenty.