It's going to have to be something BIG. And after everything they've been through together (starting with, say, the death of Gwen), I can't imagine anything that would be big enough to indefinitely postpone the wedding.
Because here's the kicker: The MJ I've come to know would have been PISSED that Peter left her at the alter. But when he showed back up and said something along the lines of "I'm sorry MJ, but Doc Ock kidnapped the mayor's daughter and I was the only one who could do anything about it . . ." she would have forgiven him. On top of that, she probably would have said, "I don't care. I just want you. Let's go to Vegas. Let's just elope."
And they would have. The FF would have flown them out and sat in the back.
In not addressing it right away, in not having a plan set in place, they have created the Alps out of an already sizable mountain. What monkey wrench could they possibly throw into the works that would make Peter and MJ, as they head towards the aisle, decide to not go through with it? Because, as they've said, it's not like they broke up then, either. They just decided to not get married. They've still been living together all this time.
I really have no idea how they're going to be able to convincingly pull this off.