Dead or Alive/WMG

There was going to be a connection between Ayane and Christie but it was dropped when Ayane became more of a Heroine
While Ayane was never depicted as flat out evil, her role was a bit more morally ambiguous namely on how Ayane was originally known as the one whom killed Helena's mother. Granted Christie has been revealed to be the one whom killed Helena's mother and was sent to kill Helena. However in Dead or Alive 2 when Helena tried to approach Ayane in rage accusing Ayane of killing her Mother, Ayane was pretty much mocking her questioning on whether if that was true. Granted Ayane wasn't flat out confirming it but she wasn't exactly denying it either. It seems oddly coincidental on how Ayane would make cover-up attempts over a murder she had nothing to do with. However by the time that happened Ayane was becoming more of a heroine especially during the modern Ninja Gaiden series. Which is the real reason why Ayane no longer had anything to do with the murder of Helena's Mother. I mean what? Where they worried about the potential implications of a teenager getting involved with a grown adult whom occasionally dabbles in nudism if Ayane and Christie did work together? (Granted I am sure certain fans wouldn't mind the "Potential Mental Imagery" there but still.)

Xtreme Beach Volleyball is real and Dead or Alive is fake.

 * The DOA tournaments were as real as professional wrestling, the DOA girls are just actors, and their backgrounds are just made up for the sake of plot. They're only fighting merely for entertainment and Fetish Fuel. In Xtreme beach volleyball the girls act as who they really are.
 * What about the male characters other than Zack?

Ninja Gaiden (Sigma and Sigma 2), DOA, and Volleyball is the past of the Halo Universe

 * DOATEC in Dead or Alive is trying to create super soldiers in the form of Project Epsilon (Hayate and Kasumi), and Project ALPHA-152 (Kasumi X and ALPHA-152). The data from the research is preserved long enough to be used by Project Orion aka the Spartan I project and Project HAYABUSA. This explains why Master Chief is so good at his job, he is an enhanced Ryu Hayabusa clone.
 * Well, Nicole's story in Dead or Alive 4 suggest that she's from the ilovebees alternate universe of Halo, so this theory may hold some ground.

The Samus cameo in Dead or Alive Dimensions is a nod to Haloid

 * The Fan Video was popular among Team Ninja, so they decided to put the Metroid stage in as a Shout-Out.

Kokoro will not be returning in DOA 5
The canon storyline will have her defeated before reaching Helena, thus she'll just have gotten her confidence beaten out of her and sent home to mummy.
 * The code of the demo seems to have Jossed this as Kokoro's name is in the demo code.
 * Indeed, something to bear in mind about Dead or Alive is that the series does include characters who either don't participate or can't participate anymore (like Gen-Fu or Ein, respectively) as bonus characters with no story. So in this case there's a chance you may both be right (she has no plot relevance and thus cannot be selected for Story Mode but will still appear as playable in other areas of the game; based on the leaked character roster).
 * Jossed to the deepest part of Hell. E3 shows that Kokoro is in the roster.