One of the great Norman Osborn stories is in 'The Amazing Spider - Man' where his transformation into the Green Goblin is explored in depth. It shows his descent into madness and his complex relationship with Spider - Man.
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.
There could be a fanfiction where Peter Parker and Norman Osborn have to team up to face a greater threat than either of them has ever faced. This forces them to put aside their differences. They might start to understand each other better during this adventure, and it could change their relationship forever.
Mentorship is also a theme sometimes. Norman, with his experience and knowledge, could potentially be a mentor to Peter in some fanfictions, despite their differences. And there's the theme of secrecy. How they keep their relationship (whatever form it takes) a secret from the rest of the world and the consequences of that secrecy.
Yes, there are many 'Harry Osborn' fanfics. You can find them on various fanfic websites like Archive of Our Own. There are different genres and themes related to his character, from friendship stories within the Spider - Man universe to his own adventures.
Sure. 'The Amazing Spider - Man: Goblin Reborn' is a great start. It gives a good introduction to Norman Osborn as the Green Goblin and his vendetta against Spider - Man. It has some classic action scenes and really sets up the dynamic between the two characters.