Composer Keiji Yamagishi confirmed yesterday via Twitter that a "new project related to Ninja Gaiden" is being worked on.
Details were scarce in the tweet, but he also said that more details regarding the project would be available in July, seeming very enthusiastic about the upcoming announcements.
It's unknown whether this project will be a video game or something like a movie spin-off, but of course it seems most likely that a new core entry in the Ninja Gaiden series is in the works.
Team Ninja, the studio behind the most recent games in the series, are currently working on a project called Nioh, which due out this year, making it unclear if they will be the ones continuing the series in the future.