Stacy, should die for one and only one purpose.
To throw Eddie Brock over the edge and into Venom. It would absolutley be the final fight we will see of the Spider-man movie franchise, basically a murder fest. Sandman should've died in the sewer (thowing peter into a depression realizing what he has done) and harry at the construction site. Venom vs. Spider-man w/ Harry doing the sacrifice thing.
Other than that, Gwen has no other reason to be killed of, we don't know her, have no connection with her, and serves of little purpose at this point into the film.
As much as I LOVE spider-man, there should not be a spider-man 4. It in no way could compete w/ the themes, and underlying stories that have occured in last 3 spider-man films.
M.J. and peter are going to end up together, etc...boring.
Unless you kill M.J. off which will really REALLY bring the series down and introduce felicia hardy.
The glory days of spider-man, end with this last movie. You can't top venom.