I seriously doubt Raimi plans to kill MJ. I don't think he'll kill Gwen either.
Months ago, Kirsten said she wouldn't return after the third movie. Back in Oct. I read an interview where she said she might, depending on the circumstances (money and story). Killing her off would be a mistake if she's on the fence.
If Dunst decides to quit the series she could be written out pretty easily. By the fourth film (if it's made) just tell the audience that she moved to California to pursue acting. Or, you could recast her. She hasn't done such an excellent job that the audience wouldn't except another actress in the role.
When Bryce was signed to play Gwen, the press release I read mentioned that Gwen died at the hands of the goblin in the comic. The person representing SONY said that charaacter WOULD NOT be dying in this film. Now, SONY could be lying to throw us off. Or, maybe Raimi brought Howard in to be love interest in future films (if the they're made) and Dunst decides to quit the series.
If both actresses were signed for the fourth film, I'd wouldn't be surprised to see the love triangle continue. One of the girls would probably meet a tragic end; with Gwen being the likely target. Right now, I just can't see Raimi killing off either character in Spidey 3.
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