Hmmm... this is a subject that i can go on possibilities over and over.
First of all, Ryu and Ayane are way better and interesting characters in NG games than they are in DOA, i honestly think that hayate and Kasumi could get that treatment.
All this talk about kasumi being innocent, is what i don't like about her character, her whole motivation in the start of the series is revenge and you want to convince me that she is not ready to chop some heads off? Also, i mentioned a lot of times in the discord, but every tiny bit of evidence points to kasumi being the stronger of the ninjas, besides Ryu of course, she is weirdly strong, is insane, if Kasumi dropped the "i don't want to fight you thing", her character would be much better.
Hayate could also be useful to something really, and one more new moveset to be included in the NG series would be really cool as well.
But see, i say this because i like what is done with ryu and ayane in the ninja gaiden series, both as characters, and i love to play with their movesets in ng3re, and i love to play as ryu in ngb and ng2 as well (still hate ng2 though), but that kind of character development is something that could be made to happen in DOA, frankly, DOA needs better characterization in general, but i still think that adding characters is nice for the variety of moveset, but that can be done the way Momiji and Kasumi are included in ng3re... just playable in chapter challenges.
Now, to resume my point. Ninja Gaiden is ryu's story, and it is about him, but i think there is space to include other characters in the story, small chapters, or even so, in an ideal world, each character having a complete campaign on their own, but honestly, is not necessary, they do nods to DOA since ngblack, and those nods are enough from a story standpoint, just add extra characters to be playable on chapter challenges and mission mode, give them interesting movesets to work with, and i'll be happy.