I really don't think it's going to be a 616 version of Miles Morales for the simple reason that it would distract from and undermine the publicity surrounding Ultimate Spider-Man. Plus, if folks were confused about how one continuity has Peter Parker still as Spider-Man while another has Miles Morales as Spider-Man, it would be even more confusing to have another version of Miles Morales in a different costumed "spider" identity.
Nor do I think it's Ben Reilly, either. Marvel has stated over and over that Ben Reilly is dead, that his story is done, and they have no plans on bringing him back.
That said, I think most likely candidate for the new Scarlet Spider is Kaine. After all, like Ben Reilly, Kaine is Peter's clone, and in this case, Kaine is more of an ruthless anti-hero, which this teaser definitely seems to imply the new Scarlet Spider is (he's even doing a kind of "Mark of Kaine" move on the guy). Plus, it's clear that Kaine's story arc is one of redemption, so having a darker version of Peter trying to find the light, as it were, seems likely, although technically, Venom already fits that bill.
However, the real interesting option could be that the new Scarlet Spider is Mary Jane Watson. After all, it's been emphasized in Spider-Island that she didn't have powers repeatedly only to discover she actually does, thus it suggests that not only will she be instrumental in helping Peter towards the end of the event, but perhaps the swerve could be that she still has them after Spider-Island is over. Plus, given the hoodie, the costume could very well hide her gender.
Then again, the other swerve could be this is really Peter himself in a new costumed identity.
Marvel obviously doesn't care about confusion if it means cold hard bucks. Waiting two years to clear up the hiccups of BND with OMIT proves that. If Marvel editorial thinks adding Morales to ASM could boost sales for it - especially as DC's New 52 will knock it out of the Top 10-20 for a few months - they'll politely ask Slott to work it in at some point. ASM is one of their top selling comics lately and they'll want to keep it in that position. Having it be outsold for a while by BATGIRL would be embarrassing.
That said you've come up with some better theories about this than I did. Kaine would be an odd choice because he's supposed to be physically larger than Peter, and he had his own distinct costume. Yes, it was very 90's (spandex, spikes, and pads with a tattered cape), but that's his look. You're right that the "dark redeemed hero" angle is being taken by Brock and Venom, but since Brock is set to join Thompson in VENOM soon, this could be Slott's way of filling that gap in his series. Kaine also has long hair, though. Making him dress like Spider-Man more directly could be kind of odd. Admittedly, he is wearing a hoodie lately as Tarantula.
Nobody at Marvel, frankly, has the stones to give MJ super powers and make her a heroine. They had their chance with Jackpot and blew it. If they weren't willing to do so then, they won't do it now. That makes as much sense as having Jason Todd return in a story, then have it turn out to be a fake-out, and then to have him seemingly return the very next story and then it turns out to be real. Oh, wait, DC did that. If MJ had super-powers, it would effectively erase a major part of why they supposedly never married, in OMIT anyway, as MJ would no longer be a damsel and could defend herself, even battle alongside Peter. Now, some of the bits with Carlie have unintentionally portrayed Peter as being so afraid of commitment and sharing power with a woman. It could make an interesting arc that all of Peter's hang-up's about women have nothing to do with his powers or enemies and everything to do with his own maturity and views, but I don't think we're heading there.
I had some long arguments about Spidey dating Ms. Marvel at the ASM topic and a few times I stated I didn't want to see it because Spider-Man couldn't handle her. He literally cannot handle being in a relationship with a woman of equal or greater physical power who not only does he not have to be the "uber responsible" one in terms of protection and risk, but HE may be the one who is the party being worried about. Yeah, Black Cat was his partner a while, but she was never as powerful even with "real" bad luck powers and some treatments by Tinkerer. Peter, somewhere, always chooses civilians to date because he can't handle fighting back to back with some he loves; he always has to keep that separate, and he does so even in EXTREME situations like Spider-Island when relaxing that mentality would have been logically efficient. Of course, fans of the marriage know that Peter can commit long term and love someone, and that MJ was a tough cookie without powers or SHIELD training. But, it isn't the same.
And it is a shame, because I do think such a relationship could be a lot of fun; like a more fun version of what Reed & Sue have but don't exploit (mostly because Reed always is in his lab being distant). Plus, having Spidey seriously date some super-heroines would be a good way to distract from the fact that he can never marry again, at least for a while. It'd beat coming up with another Mary Sue cipher once Carlie runs her course.
Peter in a new identity would suck, but...who knows. We did have IDENTITY CRISIS and those four identities less than 14 years ago.
