Despite the fact that comic book deaths are almost comically undefinitive, with major and minor characters coming back from the Underworld as if it is merely a holiday camp, Norman Osborn is about as dead as conceivably possible in Webb's Spider-Man universe. Ignoring the problems with his illness (why did Harry's condition degenerate in a matter of days compared to his father's decades? Why didn't Norman just put the Green Goblin suit on if it was secretly a medical marvel?) it looks like he's conclusively gone. That means no old Green Goblin, and no Machiavellian scheming from Spider-Man's most famous antagonist, with Harry Osborn being handed the reigns as the main big bad by the end of The Amazing Spider-Man 2, with his scheming with Mr Fiers seemingly being the symbolic transfer of power. Harry is in charge, and if Norman's head is indeed a popsicle somewhere in Oscorp, then he's not going to be coming back soon, since it would immediately dethrone Harry in a way that would be catastrophic for his impact as an ongoing villain.