Marc Webb said he wants to explore Oscorp further in the franchise and the idea that all unlikely things come from Oscorp. With that being said, the villain of the second movie will most likely be created by Oscorp.
They should save the Green Goblin and death of Gwen for TASM 3. Introduce Norman in the second movie and continue to develop him as a corrupt businessman. The great thing about the Green Goblin is that he is two villains put into one. There is the Green Goblin, who battles Spider-Man head on, and then there is Norman Osborn, who is the corrupt businessman plays a role similar to Lex Luthor. There are stories in the comics where the Green Goblin is the villain and there are stories where just Norman is the villain and the movie should show that by having him be responsible for the villain in the second movie.
The first movie already established that Osborn is a corrupt man who isn't afraid to get his hands dirty to get what he wants. The second movie should develop that even further by introducing Norman Osborn and having him play a bigger role. And then he becomes the Goblin in the third film and kills Gwen.
And for those saying they shouldn't kill Gwen in the third movie because it would be a bad way to end the trilogy, who says it has to end at a trilogy? Yes, Sony did say they planned a trilogy but that is always how Hollywood works, especially with superhero movies. First, they plan the first movie out. If the first movie is successful, they plan 2 more films and you get a trilogy. And if all 3 films are successful, they plan another set of 3 films. The old Spider-Man movies followed this same trend too, except that Spider-Man 4 to 6 fell through due to Raimi and Sony not being able to come to an agreement. If all three movies do well at the box office and are well received critically, you can bet they'll continue the franchise. And even if they do reboot after the third movie, we'll probably get a loose continuity.
The Oscorp tower almost made it in Avengers but the idea was dropped because the design for the Oscorp tower wasn't finished by the time Avengers started filming the third act. Which shows Sony and Marvel/Disney are willing to work together and makes it possible for this Spidey to be integrated in the MCU. It's unlikely the franchise will be rebooted again after TASM 3.