Well, it's official. Peter Parker is dead. The Ultimate universe version of him, anyway, which was always my favorite. I've done a little bit of work looking around to see what the Internet speculation is as to his replacement. A few comments:
1) Curiously, there was a somewhat forced and contrived story in the "regular" Spiderman continuity that ditched his marriage to MJ via a time-traveling reality altering Faustian deal kinda thingy. Marvel said this is that Peter as a bachelor again opens up more possibilities for different kinds of stories. Specifically... the exact same kinds of stories, in many ways, that Ultimate Spiderman could already tell. So... they killed Ultimate Peter Parker. Hopefully that's because there's a plan behind Ultimate Spiderman that'll blow all the fans away, but what an odd thing. It's almost as if the Amazing Spiderman folks were jealous of the success of Ultimate Spiderman, and wanted to crash his party, meaning that Ultimate Spiderman had to find a new playground to run around in. Weird and paranoid? Possibly.
2) Lots of folks talk about Ben Reilly becoming an Ultimate version of the Scarlet Spider. Clearly, including Ben Reilly at all, in the role that they did, was more of an in-joke than anything else. Having him don the Spider-threads at this point would seem stupid beyond belief. I think that sounds very unlikely, yet it seems to be a popular speculation by fans. Probably because they can't think outside the box enough to think of anything other than a rehash of the 90's Clone Saga. Blegh.
3) The Peter Parker clone with the Scorpion tail graft up his spinal cord is another popular one, just because hey, he's the only male Peter Parker clone left. That doesn't make him a good candidate, though.
4) Ultimate Jessica Drew is another popular one, but I'm convinced that the new Spiderman will remain a Spiderman and not a Spiderwoman in drag. I do hope that she steps up and plays a more prominent role in the stories as they continue to evolve, though. I like Ultimate Jessica Drew a lot.
5) My own personal theory: somehow there's going to be an Ultimate Miguel O'Hara! He's a reasonably popular character and variant of the Spider-man archetype, and I think letting him have a run in the suit, with a suitably appropriate backstory, and a suitably different take on what it means to be Spiderman, would be fun. I think the thing Bendis needs to avoid is making this new Spiderman try to slip too much into Peter's role; he needs to be an all new Spiderman, with his own set of friends and whatnot.
6) And of course, the more cynical and jaded of us comic book fans don't really believe that Ultimate Peter Parker is dead for good anyway. Although to give the "Death of Spiderman" story sufficient gravitas he's gotta stay dead for a little while, at the same time, Spiderman who's someone other than Peter Parker just doesn't seem right in the long-term.
7) Maybe they're will be four replacement Spidermen; a cyborg Spiderman, an alien Spiderman, Spiderboy and a black guy in an armored Spider-suit... Nah.