Here's a go on what I think the story should be:
After the final encounter with the false Phase 4 in the laboratory and after defeating the clone once and for all, Kasumi begins to go after Donovan once and for all. After honing her skills for 6 months, Helena joins the ninjas on the plan to draw Donovan, Phase 4, Rig, and Christie in the DOA6 tournament, with Helena entering as well to finally settle the score with Christie.
Here's my short story summaries for everyone:
~Deceased.
~Enters after hearing Raidou is in the tournament. She has no idea he doesn't have his memories or that Donovan is behind his resurrection so she blindly goes after him to exact revenge
~Enters to thwart Rig after hearing of his dubious nature. Also enters the DOA6 tourney in hopes of protecting his daughter Tina from anything sinister.
~Receives a letter from a mercenary, challenging him to a deathmatch in DOA6. Enters the tournament to see whom this man is, possibly an old rival?
~Drifted away from Eliot after the events of DOA5. With no money or any job, he enters DOA6 in hopes of reuniting with Eliot and for financial security(Or funding for his alcohol habit...)
~Is sick of Donovan and has turned traitor, having jumped ship from MIST after the collapse of the false Phase 4. After spending months in hiding, she hears news her prey Helena is participating and hosting the 6th DOA tournament. She enters to finally bite and eradicate her prey once and for all before she can get close to Donovan.
~Having learned some new techniques from Gen Fu, Eliot is at peace but his master is becoming weaker as old age is catching up to him. Wanting to show his master that he is indeed ready to graduate from apprenticeship, Eliot spends the next months training harder than before. His body and mind at peak condition, he decides to enter DOA6 in hopes that if he wins, he will make his master proud.
~Enters to also pursue Donovan, but also as a guardian angel of silence to Kasumi. Although he cannot speak to her because of clan code, he will still ensure that she live to fulfill her task.
~Hosts DOA6 to draw Donovan and his weapon Phase 4 out in the open, however she is unaware that blood soaked hands are watching her ready to kill her if she gets any closer to Donovan...
~Coming in second from the last tourney, she enters yet again with a stronger vigor than ever to win this competition! To make it like the old days, Leifang and Hitomi both promise each other that they will make it to the finals to see which one is stronger.
~Hears of the DOA6 tournament, and she excitedly enters ready to try her secret ability! Little does she know, MIST has been watching her for quite a while, perhaps capturing her and cloning her ability could benefit the development of Phase 4...
~ Still annoyed at losing to Rig, he Enters DOA6 in the hopes of defeating him in a rematch. Little does he know that Rig is more complex and dangerous than he thought. Will he be entangled in this mess?
~Having honed her skills for the past 6 months, Kasumi has tossed all inhibitions out the window and has evolved into a warrior not afraid to hurt those that stand in her way of her clan's safety. She Enters DOA6 to finally defeat Donovan, but a new threat is arising... It doesn't look like her battle with clones is quite over yet.
~She didn't win the last tournament but her conversation and sparring match with Helena has inspired her to continue training against her mother's wishes. Hearing of the DOA6 tournament, she runs away from home clad in only her kimono and with a couple belongings, she travels to enter the DOA6 tournament to reunite with Helena. Having connections to DOATEC and MIST thanks to her mother's job she mentioned previously, Kokoro will be able to travel and return home safely thanks to an unknown sponsor.
~Badly hurt after the accident in the lab, Lisa made a speedy recovery thanks to the DOATEC facilities Helena supplied. Having an epiphany, she resumes turning against Donovan but acts as a fly on the wall, posing as a scientist for MIST as she gathers information for Helena. Hearing of DOA6 and the possibility of Donovan and his associates involvement, she joins the fray posing as La Mariposa to find more leads....but unbeknownst to her, Donovan is well aware of her treachery. It is only a matter of time before her "hot beats" will burn out if she isn't careful.
~Leifang once again lost to Jann Lee but she finally accepted her defeat, realizing she was strong in her own right and she didnt need to keep trying to prove that to Jann Lee. Resuming her studies and practicing Tai chi, she trains along with Hitomi as the two plan to enter DOA6 to settle which among the two of them is stronger.
~Leon was recently plagued by nightmares of his dead love Lauren... Her spirit wasn't able to rest, he could sense it. Her death wouldn't have happened if it weren't for THAT man... DOA6... Maybe DOA6 will finally let him exact revenge once and for all for the loss of his beloved. Challenging him, he prepares for his toughest task thus far: Killing Bayman. Maybe then can Lauren be at peace and he can finally rest.
~Helena's servant, having recently just graduated from her private high school, she spends her days catering and assisting her master in DOATEC. Having been in gymnastics and dance classes from a young age, she's over the years been studying systema, and learning how to use her lovely, graceful movements to weaken and hurt her foes where it hurts most, all enhanced by her somewhat devilish nature and sadism hidden by her cute face and small stature. Hearing talk of the DOA6 tournament, she enters in the hopes of proving size doesn't matter to her adversaries, as well as to finally sate her sadistic thirst for violence.
~After the exposure Tina gave her, Mila got alot of MMA offers and deals and is on her way to becoming a fighting star! She still keeps in close contact with Tina, who's also got an itch to fight once again. Wanting to feel the thrill of DOA again, she enters the next tournament, ready to make her mark on the world!
~Enters to test her abilities out further, as well as to assist her master Ryu in watching MIST.
~Growing bored of her Tengu world, Nyo wanders into the human realm, wanting some entertainment. While not invited, she sets her sights to crash the DOA6 tournament to see if anyone will be able to handle her. She also has a surprise for Ryu, the man she recently learned was responsible for the death of Bankotsubo... She does not plan to share the same fate as Bankotsubo.
~Having been finally programmed with the last steps into her cloning, Phase 4 is the final product of the project and Donovan is ready to unleash his new queen onto the chess table. As a tear falls from her eye, she walks out onto the arena, with her freedom being so close, yet so far away. The tragic weapon is ready, with all that she's learned from the 4 ninja at her disposal.
~Enters to not only assist Ryu, but is also interested to see if there's any humans worthy of a challenge.
~Having been revived months ago, Donovan has closely observed and watched his new pawn. Without his memories but with his brutal lust to fight and hurt still engraved in his inner machinations, Donovan decides to let this flawed, but still intriguing dog loose in the DOA6 tournament, knowing full well Ayane and Kasumi will not be prepared for this shocking revival to meet them again face to face after two years.
~Rig continues to help his dad Donovan out with his MIST operations. But he still needs some associates, with Christie gone and Lisa recently being discovered to be working as a spy, Donovan chooses to let Lisa be, they'll eradicate her when the time is right. However, Rig sets his sights on a new guy, Diego.
~Having no desire to enter DOA6 and knowing Kasumi will take care of Donovan, Ryu continues his watch on MIST. However recently some weird wind activities have destroyed numerous buildings and monuments.... This isn't anything ordinary. Suspecting this to be the work of a tengu, he has no idea that the beautiful threat behind this is out for his head, he must act quickly before the threat thwarts Kasumi in her mission to go pursue Donovan, or before this threat will have her way with the legacy of Ryu...
~Wanting to run for governor, Tina uses her wrestling career to get supporters and attention, as her crowd approval soars up, but with months passing by and the polls drawing to a close, she needs something big to help her. DOA6 is the perfect opportunity for her to get her name out there! Little does she know of the dangers that lie hidden in the remnants of DOATEC, but surely Tina can emerge unscathed.
~Having matured from the events of the last game, Zack is now devoted to helping Helena and making her happy as the leader of DOATEC. With her being a contender, he Enters as well, wishing to test his skills and see if he's still got it. Surely the winner of DOA4 can win??
That's all I can muster, forgive me if my ideas are bad xD