I've often thought a good plot line would be if his deal with Mephisto that erased his marriage did more than just have personal emotional consequences something I think would be good would be for the original Gwen to return and for it to be revealed that Mephisto brought her back but as a sort of slap in the face to Peter and she came back crazy or came back not all together. I can absolutely see Mephisto doing that and revealing it to Peter with a "Oh you didn't read your whole contract did you?"
Essentially this is what I'm thinking someone new steals the Green Goblin title from Norman and not only goes after him but Spider-man as well taking over the city's criminal underworld as the new Kingpin. Spider-man tries his best to fight this Goblin only to have things escalate and get his past failures thrown in his face as some sort of cryptic clue to the new Goblin's identity like he's led to the place where his Uncle Ben was shot and finds his Aunt May held hostage by a super villain and the theme continues in some sort of twisted gauntlet with the new goblin reenacting the deaths of his loved ones using people he cares about that are still alive with each challenge having a twisted theme to it like trying to teach Peter he can't save everyone or trying to make him kill the bad guys (with him finding another way of course) to save people and such like. Then at the end Peter is led to the bridge where Gwen died and finds Norman held hostage by the new Goblin and learns it's a resurrected Gwen that's out for revenge and demands that either Peter kills Norman or watches as she does.
I'd honestly love a story arc like this because I think it'd be very emotional but at the same time I'm not sure how Norman would react to it I feel like depending on which personality is driving be it the business man he'd be pissed at having everything from his empire to his goblin gear be stolen from him and taken hostage. At the same time I feel like if Goblin was driving then he'd be getting a kick out of the whole thing while being slightly irritated that he was being impersonated but at the same time if he learned it was Gwen behind everything I can see Norman/Goblin just egging her on and just enjoying the ride maybe even giving some mild humorous criticism.
All in all I'd really like to see Spider-man handle a stand off at the bridge where things are completely messed up with Norman as the hostage and Gwen as the Goblin trying to force Peter to make a choice and berating Peter for letting Norman live all these years. Pretty much something like this.