One of the stages in DOA2 Ultimate had a Ninja Gaiden ad as a billboard.(not to mention it was the debut of the Legendary Black Falcon in DOA) Not sure which came first though.That makes me think about Team Ninja.
We had DOA 1, 2 and 3. And then they made Ninja Gaiden and after that, Ninja Gaiden Black, in Ninja Gaiden Black there is already DOA 4 trailers, and then, DOA 4 is released, and after that Ninja Gaiden 2, then, Ninja Gaiden 3 and DOA 5 are released in a similar time frame as well. Dead or Alive and Ninja Gaiden have been "walking" together for a while now. I hope we get a new NG announced around next year at least, that would be nice.