My theory is a bit different. Peter feels responsible for Alpha. In ASM 699, Peter will be fighting Doc Ock, but Peter is holding to his values and not killing him...but is steadily losing the battle because of it... In swoops Alpha to save the day and kills Doc Ock in the ensuing melee. Probably by Alpha overloading his powers and exploding or something. Peter is pretty seriously injured. ASM #700 opens with an injured Peter also dealing with the guilt of someone being killed by a sidekick under his responsibility, so he decides to call it quits, at least for a while. But a new Spiderman will emerge, perhaps under Peter's tutelage. This new Spiderman will carry the mantle for some 20-30 issues while Peter recuperates and deals with his crisis of conscience. It has to be someone a bit unique, with an interesting personality and storyline. My guess - Uatu Jackson! Uatu discovers that Peter is Spidey because this guy in Horizon who has been sabotaging things leaves Peter's lab unlocked and Uatu stumbles in there. Turns out that Uatu is an interesting young guy and there are new characters and storylines. He builds powers based on technology, not innate powers (he's a genius after all). Peter gets back with MJ, setting up stories for what happens when he comes back 20-30 issues from now. The only downside with this theory is that Uatu is too similar to the Spidey character in the Ultimate universe.