I doubt TN is using that poll as a blueprint for anything at this point. When I look around YouTube and other social media comments, I've observed more people requesting Rachel than any of the other girls. I see mainly Rachel and Lisa if I'm being blunt. The fact that she has a brand-new model for NGIIB also promotes the idea of relevancy. Meaning out of all the missing girls, Rachel is the first one to reappear in a significant number of years with an updated take on her body.
Mila's popularity on social media has died immensely, and while I can't rule your opinion out completely, I can say with certainty that more people talk about Rachel and for good reason. She's far more attractive than Mila (hot take from some I'm sure) by every plausible standard; especially by the merit of being something different in the DOAX game. She's thick (which TN will do from time to time given the look of Sayuri) she's also got the third or fourth largest bust I believe and could present an interesting storyline. She's tall and has a far more iconic outfit that has yet to release in DOAX, but very well should at some point.
Mila's story is so overdone by the equally ambitious Tina and Mila's base outfit is extremely bland and bourgeois.
You also have to factor in that NA players feedback is starting to mean more to the market than it did several years ago. It's very popular on Steam and people are completely open to being a YEAR behind in content to play it on Steam and not awful DMM.
So, I hope that you can look past my hot takes and see my arguments as very possible. I think the next OG girl is going to be Lisa, NiCO or MY GIRL Rachel. The biggest case for Mila I can make is that she's a tomboy and that rancid poll.
OH and one more argument I can make is Team NINJA is calling this the year of NINJA Gaiden. They are going to want to celebrate it in every marketable and fathomable way. What better way to market even NGIIB than to release Rachel.
Whether it's Mila or Rachel really doesn't matter to me. As long as we actually GET at LEAST one cut girl in there, I'll be fine (heck, I'm hoping we get most if not ALL the cut girls in this year, including NiCO).
So far as Mila's concerned, to be fair, we did technically have a tomboy character in the game (since Lobelia EXPLICITLY referred to Fiona as one in that rock-climbing episode), and there's nothing against adding a different flavor of tomboy in there (just ask Momiji, Sayuri and technically Tamaki, who are three different takes on the Big Sis archetype. And that's not counting how we technically have five different Tsunderes in the game [Ayane, Nagisa, Elise, Amy [and that one was specifically meant to be written as one, BTW] and Reika]). As far as Mila's story, what's to say we can't have her, say, going to the island and trying to participate in the festival with emphasis on the strength part in addition to beauty. That would be a twist to the norm (as even Hitomi was implied to place some value on the beauty requirements even if she wasn't quite sure how to reach that threshold). Oh, and then discovering Tina's there as well.
And as far as outfits are concerned, I think they can figure something out for her. Maybe something sporty similar to Reika.
That said, I definitely agree Rachel ought to take precedence this year, mostly because of the whole Year of Ninja thing they are doing (plus, it WOULD make a good promotion of the upcoming Ninja Gaiden games either way).