Well, Norman Osborn's death in the comics is a result of a series of events. It involves conflicts with various superheroes and his own evil schemes backfiring on him.
Another possible story is a time - travel based one. What if Mary Jane Watson is sent back in time and meets a younger Norman Osborn before he turns into the Green Goblin? She could try to prevent him from making the choices that lead him down the dark path. This would create a complex dynamic as she has to navigate the past world and also deal with a Norman who is not yet the villain she knows from her own time.
Some Reddit communities dedicated to Spider - Man or fanfiction in general might also be a great source. Members often share and discuss their favorite fanfictions, and they might be able to point you in the direction of some great Peter Parker and Norman Osborn stories.
Their relationship could be developed uniquely if Norman Osborn, in an alternate universe, was not a villain at all. He could be a successful businessman who meets Mary Jane Watson at a charity event. They start off as friends, sharing a common interest in helping the less fortunate in the city. As time goes on, they develop deeper feelings for each other, and Mary Jane helps Norman deal with his personal demons, which are more related to his family and business pressures rather than super - villainy.
Well, it happened when the symbiote found Cletus Kasady. This combination of a powerful symbiote and a deranged mind gave birth to Carnage. The symbiote's influence amplified Kasady's evil tendencies, making him a truly terrifying force.
Carnage is a major villain. He's basically an even more dangerous and unstable version of Venom. His symbiote gives him enhanced strength, speed, and the ability to shape weapons and appendages. He's known for his psychotic and unpredictable behavior.