I'm actually not too disappointed that Itagaki decided to end Ninja Gaiden. I feel that more developers should consider pushing towards the idea of definitive games, meaning making the game of forever instead of the game of 2008. The sequels were entertaining though, and at the same time, I don't wish for it to end either. Still, Itagaki doesn't even own NG, and just because he decides to put a stop to it, doesn't mean Tecmo won't. The series is too well known, so I'd imagine someone else will take his place. Then again, I probably wouldn't feel as confident in a new NG game if he wasn't in the helm. D:
A WWII game is rather vague. It seems a lot of people just think of FPS and the occasional RTS. That doesn't mean it can't be original though, and it's when you do a literal interpretation that it's stale. I, myself, don't feel particularly interested in what he does next.