I'm just starting to worry about the integration of the existing characters. I for one didn't play the xbox DoA's as much, nor 3DS dimensions. For me some character just fell off in the story, counting Kokoro, Lisa and so on.
So I feel that DoA6 needs is a solid story more than anything in order to grow. The best selling fighters (not to be confused with the best fighters) has a very good story, and makes "a new game every time", as in they reintroduce the cast in a way that both old and newcomers get an idea as for their identity. This shouldn't be hard in DoA with their lovely diverse personalities. It may also be particular important here considering they don't have starpower like the DC comic or long history legacy. Mortal Kombat and Injustice has done this very well. Another technique or alternative is giving many different taunts, entry and finishing poses. This tells a lot about the character.
Also I agree with the girl/boy ratio being somewhat close.
Currently it is 15 girls :14 boys, counting Ein/Hayate as 2 persons (it might happen) and alpha as a girl. If the VF characters vanish it is down to 13:12. Then again the girls most likely bring in more cash, considering they get more costumes and DLC. So I'd expect TN to bring in more girls than guys next round.
If you give alpha 152 a male form as well, you cheat and make it 12,5:12,5 as a basis.
However, I actually don't think the VF characters will vanish. I think both DoA and VF gain benefits with a small "demo window" of characters between themselves. Because if I ever manage to do any of the VF characters well in this game I'd probably buy the next VF, and so on.
I also think they'd want to bring in a guest character from some other series. It often help both parties, and TN has been doing it a lot lately, and they should, as they are not the powerhouses Capcom, Nintendo or Netherealm. I don't view it as unlikely to band an alliance with KoF and maybe bring in characters like:
Why did Mai's image get so much larger?!
Anyways, KoF is not the only series of fighters they may bring in an alliance with.
Skullgirls I am pretty sure would be in awe of any such opportunity, though they might be a little small as of now.
You also have Soul Calibur, which is known for bring in guest characters themselves.
Characters easy to implement (as their style is close to fist fighting):
Ye, was going to do more, but seem to be out of picture, but you get the point.
Anyways, alliances is good and all, but doing 2 new original characters would be good as well, one of each gender!