My far-fetched theory is that something goes down and Peter/Spider-Man is believed killed. He actually gets thrust into a Quantum Leap style storyline jumping from various realities Ultimate, House of M, pre-OMD, zombie universe, etc. The world he leaves behind must cope with his death while he tries desperately to get back home to the 616. Or something. It's a long shot but it's mine ;) That aside, if someone else is gonna die I'm gonna say Carlie.