She will have new moves for sure, many even if Ninja Gaiden 4 comes out before Dead or Alive 6( Momiji's movelist is mostly her Ninja Gaiden 2 moveset but without the weapon she uses in that game). Story-wise I don't know what she would do though.
Momiji does put people to sleep though. If people had insomnia, Momiji would fit right in that category to cure it. Gen Fu could be added to that as well.
Rachel is the queen of it. I honestly don't dislike her style it's pretty cool and unique, but you're going to get tired of anything after seeing it 100 times especially if it's killing you.
I don't really care about how boring the character is, a lot of characters can go into that category; it's DOA. But she has all the "right" tools. She's not the greatest, but she's not bad is basically what I'm trying to say. She's just right. A lot of characters in DOA have to play a lame neutral game, unless you're a VF character or Helena/Christie.