Spiderman is Marvel, no doubt about it. He's got that classic Marvel charm and his storylines are deeply integrated into the larger Marvel comic universe.
It's hard to give an exact age as it depends on the specific storyline and continuity. But generally, Spiderman was likely in his early to mid-twenties in the 1990 comic.
The Green Goblin is a classic choice. His maniacal nature and his flying glider make for some exciting action scenes. Mary Jane could also be included. Her relationship with Spider - Man is an important part of the Spider - Man lore. And don't forget about Aunt May. She can add some heartwarming and sometimes comical family - related moments to the fanfic.
Maybe a plot where Spider - Man has to face a new villain who has the power to manipulate time. This would make for some really interesting battles and strategic thinking on Spider - Man's part. He would have to find a way to defeat the villain without getting caught in the time - warping chaos. Also, a story about Spider - Man traveling to an alternate dimension where his powers work differently could be cool.