The way I see it, it depends on what kind of person you think Slott is.
One of two things are going through his mind...
1. MArvel's desperate, I can bring the marriage back, and even my harshest critics will love me.
2. I am going to give one last "F. you" to to those fans who have been bugging me, and kill MJ.
Which type of person do you think Slott is?
The solicit for #800 asks "who dies?" That is what made me think it was MJ dying... however, it could theoretically by Aunt May dying as a result of the deal being reversed.
The greatest mystery in comics right now is, who is Dan Slott?
I'm betting on 2.
Slott machine is well remembered by me as someone who can't take criticism and I wouldn't be surprised if he is planning one last giant middle finger to his critics.
However there is also the revolving door of death for all except captain mar_vell. The cloned Gwen back, MJ was thought to be dead for a time, aunt may was retcon revived after dying of old age.....