So, I decided to go ahead & open up a can of worms with this one:
Kasumi vs Ayane vs Momiji.
I think it should happen somewhere down the lines. This should be a triple threat rivalry. It may not be now, but in the future, it should. Now I know that some will be like "Oh why should Momiji be a rival to them both? She has no purpose", but the fact is she does have one & can rival them in the storylines as a ninja from the Busa village. TN can throw in a curve for it to happen. Then some of you are like "Kasumi & Ayane shouldn't fight". lol Yes, they should. They don't have to "hate" each other now since the storylines are changing tides (just like Momiji doesn't have to hate them), but they have to fight each other. They're destined to fight each other, if you don't realize that by now, then you need to. You can't have a good action-packed game without a rivalry. Look at Ryu & Ken from SF. They're friends but also rivals. Sonic, Silver, & Shadow, same thing. Heck even Ryu & Hayate are rivals but great friends.
I say let the female ninjas duke it out! And no I'm not talking about looks or anything like that -__-