I'm pretty sure I know what is going to happen. [ Keep in mind. Spoilers of recent events. ]
1) One of the most likely scenarios would be something with Jackal and Kaine. (Here me out before you rage. Horrid memories of clone saga, I know. We all have them. Not saying I agree with what they did back then, just seeing what they may be going towards.) After the events of Spider Island which was the biggest Arc besides the recent Ends of The Earth. At the end we know pretty much 3 things. The Jackal is still alive and well. Kaine and Madam Webb have something going on in the background with how she won't tell peter why he needed the suit he made during Spider Island. And Jackal is harvesting The Spider-Queen's dead body. I expect something with one of them. They are hinting at it. Kinda obvious that something is coming soon with them and 700 seems like the nice way to do things.
2) Another likely scenario how ever horrible is something with Jackal again, and a Gwen Stacy clone. Why? Marvel has a tendacy to tie comics in with the movies, so when new readers jump on they are not confused. (Continuity be damned.) Dan Sloth saying "I am breaking the comic pinata and all the candy is coming out. And it is not coming back together." I think implies that he is going to be break something obviously. They are saying it is gonna be huge. Yada Yada. If anything this would be it, and the ONLY way to go about doing it with out pissing off a large majority of the current remaining readers.
3) Aunt May dies. Seriously? She has been so close to death so many times. Old-Hag is like 90 something. Seriously, she needs to die. Another tragic event would obviously be good 'ol peter parker luck.
4) Something with Venom. Venom has been involved in alot of Arcs through Flash Thompson lately. Certainly no big villainous acts lately with him. So....Seomthing needs to happen with him. People have been wanting to see Venom go all out again. 700 seems like a big way to do it.
One of these is pretty much gonna be the "basis" for 700/leading up to it at least in my opinion.
Doctor Oc is pretty much dead after the end of Ends of The Earth. Electoro? Shot him into outer space.
I think it might be Peter finally killing someone (a villain, maybe even justifiably) out of pure rage and then having to deal with the aftermath of his actions. He's come close in a few of the recent arcs and he seems to be getting pushed further and further over the edge, and with Kaine (now with sanity) showing no compunction over it, we know it's in his basic make up. It'd make a very interesting story arc rather than another batch of "uh oh, i can't let my smart girlfriend know i'm spiderman, woops she found out anyway" (i really disliked Carly as a character) story arcs.