I personally don’t get the importance of their nationalities since it really doesn’t matter where they come from. I think Lisa got added to DOA series because they couldn’t add her as DLC to DOA3. Such practices were rare during that time period so it kind of would’ve made sense that they would add her in upcoming game which was DOAX. Lisa would’ve been in the next upcoming DOA regardless if it was DOAX or DOA4, these VV girls look cool and such but we really do not need them in DOA6. Not everybody in the DOA universe has to be a fighter. I wouldn’t mind them being appearing as background characters.Eh, I don't mind NiCO too much, though I do wish they made her older so her being a key scientist in MIST would be a bit more plausible, since that takes a lifetime of training (yes, Lisa also was a bit young to be a top scientist, but at least you could argue that she was possibly going through training.). I mean, Nagisa proved you can make a flat chested girl WITHOUT needing to make her 18 bare minimum.
Also, just as an FYI, even if we were to get the DOAXVV cast on DOA6, the majority of the female cast would still be older than 18. By my count, we have Ayane, Fiona, Luna, Honoka, Marie Rose, Misaki, and NiCO. Seven girls total. As such, not counting Momiji or Rachel, since their status isn't even clear as of yet, we'd have Nagisa, Tamaki, Tina Armstrong, Lisa Hamilton, Helena Douglas, Mila, Leifang, Hitomi, Kasumi, Kokoro, Nyotengu, and Phase 4 (technically, due to her being a full-grown Kasumi clone), leaving 12 girls who are older than 18 (19 at the absolute minimum). Assuming Momiji and Rachel ARE in DOA6, that would give us 14 girls who are older than 18 (well, okay, 13, technically, since Rachel's age is unknown).
As far as why they should be added in, do I really have to remind you of the case of Lisa Hamilton? Unlike the other girls in the roster right now, she neither debuted in one of the mainline fighting games or even in Ninja Gaiden, she debuted in an Xtreme game, the first one as a matter of fact. However, she was added to the roster by DOA4 with the Luchadora style. If they can find a way to make Lisa into a fighter despite hailing from a beach game, they most certainly can do the same for the DOAXVV girls. And besides, adding Luna and Fiona would at least solve the issue about the girls' origins, considering how it was stupid that DOAXVV just neglected to mention their nationalities.
Out of all the VV girls I think Nagita is probably the one that I can actually see fight since she wants to protect her little sister Misaki. We could get way better characters for DOA6, I would like it if they added one Male and one female character as DLC post launch.