Hideaki Wakui re-earned his free agent option today. He earned and exercised his initial domestic option after the 2013 season. He re-earned the option (which now also includes international rights) with four years of service time on the active roster.
[UPDATE 9/5 @ 3:05pm]
It is uncertain if Wakui signed a one- or multi-year deal before the start of the season. Regardless, he likely has the ability to opt out and exercise his option if he so chooses. The Marines would like him to return next year and are okay with him exercising his option and then ultimately staying, something they normally do not do.
Source: Nikkan Sports 9/5/2017
[UPDATE 9/6 @ 1:37am]
Wakui has mentioned interest in the Majors and could head overseas after the season. However, if he decides to stay, a number of teams could be interested, like Chunichi, Yokohama, and Hanshin.
Source: Tokyo Sports 9/5/2017
[UPDATE 9/10 @ 10:52pm]
Scouts from at least four MLB teams (Giants, Mariners, Phillies, and Cardinals) were at Wakui’s last start on September 9.