The Nippon Ham Fighters announced that they selected Kitashiroshima, Hokkaido as the location for the new stadium. Construction is slated to begin in May 2020. If everything goes to plan, the new facilities should be ready by March 2023. Additional information will be released on November 5.
Source: Nippon Ham 10/31/2018, Daily Sports 10/31/2018, Sports Hochi 10/31/2018
[UPDATE 11/5 @ 4:26pm]
The Fighters released additional information about the new stadium today. Some highlights:
Source: Nippon Ham 11/5/2018 (in English)