I agree. Everyone tries to cite Rachel's death in TDK as the "It's been done before." moment, but Rachel died at the hands of the villain, and while tragic, it doesn't even hold a ****ing candle to the death of Gwen Stacy.
This is particularly why I want her death to be in the final film of the franchise. Sure, the trilogy would end on a somber, almost depressing note, but it would set the film apart from others forever, really. It would shock the general audience after letting them fall in love with the character for almost 3 films. I think it would rock the cinematic world just like it did the comic world in the 1960s.
Picture this:
It's TASM 3 and Norman Osborn has been set up as the ultimate villain for 3 films spanning nearly 6 years. The hero's love interest is being held captive at the top of the George Washington Bridge and the audience is gearing up for another heroic save. The Green Goblin drops Gwen, it seems as though Spider-Man has saved her. He pulls her up but she won't wake up. He begs and pleads for Gwen to wake up but she won't. The Goblin arises in the background laughing. He taunts Spider-Man/Peter for being so smart yet not being able to realize that he had just caused her death. Peter, so saddened that he can barely move, swears revenge on the Goblin as he escapes. The scene ends with Peter holding Gwen in his arms.
Obviously the movie would end after Spider-Man found Goblin and beat him down. At this point Mary Jane should have been introduced, but not as a love interest. Simply as a friend to both Peter AND Gwen. There could be a scene towards the end of the film after Spidey's encounter with Goblin has been resolved where Mary Jane and Peter are comforting each other over the loss of their friend. A "1 Year Later" screen could show up and Mary Jane and Peter are studying together. Peter eventually asks Mary Jane out on a date and she accepts.
That's my ideal ending to the trilogy as it is very bittersweet and would leave a mark on CBMs forever. It also reminds me a bit of the (500) Days of Summer ending which would be a cool similarity between Marc Webb's works.