New Team Ninja boss Yosuke Hayashi has told Kikizo that Ninja Gaiden will almost certainly be continued as a series.
"What the gamers are looking for is going to be the next chapter - the future of an outstanding series. We feel that we're in a position of being able to make that call... That calling is already there - we feel it".
Tomonobu Itagaki, the previous Ninja Gaiden dev head, said in May 2008 that the second game would finish the IP.
Apparently not. It gets better: next time it's going to be multi-format:
"We're going to have to push ourselves to the next level, and we are better off giving both audiences an entirely new game," Hayashi added.
There's a full interview here.