According to the April 2016 issue of Young Ace magazine, the manga Boku Dake ga Inai Machi will soon have a spin-off release after the final chapter ends. The spin-off manga is scheduled to be published on the July 2016 issue of Young Ace which will go on sale starting June 4. The spin-off manga titled “Boku dake ga Inai Machi Gaiden” will include additional stories which will explain things that were not discussed in the original story.
Boku Dake ga Inai Machi’s 8th volume which is also the final volume of the manga is scheduled to go on sale on May 2. The current anime adaptation Boku Dake ga Inai Machi which is scheduled to end this March will have an original ending taking into account the whole story of its manga counterpart.
Source: Comic Natalie