The next time Aunt May dies she better just stay dead. Don't bother killing her or don't bring her back if you do. Nothing against the character but she has come back to life almost as much as Kaine.
As for MJ? It would be really ballsy, and cause a huge uproar. Thing is that a death and especially one for a long time character needs to have weight to it. You can't just kill her off and move on. She plays such a huge role with Peter despite them no longer dating or being married. It just doesn't seem like something worth doing.
However I promise you that if someone wrote MJ died, she was shot sixty times in the face, partially eaten by a supervillain, drowned, incinerated thrown into space in a vibranium/ adamantium coffin and sent into the sun with her soul being destroyed and wiped clean from history. A hack writer will bring her back and claim it was an actress, all a dream, or get Mephisto to warp reality. I am so not kidding.