It looks nothing like Dru, it looks exactly like that woman who won the contest
How can you tell? Her face is almost completely covered in shadow. All that we can see from this is that she has long dark hair and is wearing a dress that looks kinda old-fashioned (I'm a guy, so I know jack about women's clothes; maybe some of the ladies can say more about what she's wearing). Also, it's not a photo, it's a drawing, so there's no way of knowing yet how Jeanty might draw Dru. His style is great, but not exactly photo-realistic. (And yes, I know Moline's doing the interiors for this arc, but Jeanty's doing the variant covers.)
And from what was shown of Robin in #10, she seemed like a force of Good; it'd be a pretty lousy thing to do to her to write her into the comics and turn her into a villain, wouldn't it? Especially considering what her husband has said about her mental illness, where it sounds like she has a lot of difficulty separating the fantasy world of "Buffy" from the real one. She might not react well to her story taking a 'Skip'-ish sort of turn. And we know that her character was a "minder", but do we know that a minder is immortal? Not yet--but we
do know that vampires (like Dru) are.
Not saying it's
not Robin, but I just think Dru is a more likely candidate to be there alongside the future's supreme badass vampire. And she's actually been a pretty popular guess on some other boards, which is why I was a little surprised she hadn't been mentioned here yet. (Other theories I've heard include Dark Willow and Amy, but I don't buy either of those; I'm stickin' with Dru.)
Edit:
Robin wasn't someone I'd even considered before, so this only occurred to me after I'd posted the above message. ^^
Anyway, the #17 variant cover is by Georges Jeanty. The issue that Robin appeared in was #10. Jeanty didn't do the interiors for that one, either, but he did the variant cover, which Robin appeared on. So there's a frame of reference for how he draws her. And while she's not fat by any means, she's not super-thin, either. If you compare the two covers, the mystery woman on #17 is
much slimmer.