If I may respond to that: The "A Month With Jana" series had a password which made it appear "exclusive" and intimate to all users who signed up for it from day 1. Nevertheless, it was open to public before the actual series had ended, which deprived it of its exclusivness and intimacy to the "original" users, since all of a sudden the whole community was part of it, even if they missed day 1. This isn't generally a bad thing, but it depends on what you're aiming at. For a series which intends to build a fictional relationship to the user I'd go with a password and a sign up date. For a tease-series, which focus is not on the interaction between "model" and user, the lack of password won't matter.
Regarding the models choice, I would DEFINITELY vote Angel Dark, although I'm aware of the "problem" you've already mentioned about her appearance in some shoots, Nezhul. Fortunately, those are commonly in the older sets of hers, so if you want to choose her and mainly pick shoots from 2008 or newer, I see no problem.
What bothers me about her is that I already heard her talking english...I don't know about others but to me it's important that the model in a tease is not only hot but also provides something which I like to call "obvious intellect", which basically means the model
could talk and act like the tease makes her (turns out it's difficult to describe, so I hope everyone understood my point
).
With Angel Dark I don't have this illusion anymore...^^
Out of the already suggested models I could imagine Kayden Kross in the series. She may seem more strongly feminine than girly (like you already said) but I think, she also appears playful in many sets and a mixture of both strongly feminine and teasingly playful could really hit it off.
And also I wanna add Georgia Jones, I don't know if she does enough hardcore (on top of that I don't know what you consider hardcore) but she has a lot of stuff out there and certainly a playful, yet strict expression.
Though I could understand that she's not everybody's choice...
http://galleries.penthouse.com/gallerie ... 1/L/02.jpg
I hope I could help a little and I'm already looking forward to your work
I have around 70 sets with her (50 of them i'd consider worthy to be used in a tease). Best sets are from x-art.com, twistys.com, atkpremium.com, nubiles.net, ftvgirls.com and digitaldesire.com. But it's ok... so i can make a series with her sometime 
all of them copyrighted ... soo... not a good idea... sorry
