They can close out the trilogy with 2 hours of the investigation into Peter's spy parents or whatever, plus his relationship with Mary Jane, plus a focus on Harry, plus the return of Electro, plus the return of Rhino, plus the creation of Vulture, plus the drama of Aunt May's nursing job, plus Peter trying to do well in college, plus Peter emo'ing out over the death of Gwen, plus Peter starting up his own business, plus Peter dealing with JJJ, plus the creation of Doc Ock, Shocker, Leap Frog, Venom, Hammerhead, Carnage, Spider-Ham, Chameleon, Hobgoblin, Vulture, Jr., Aunt May Venom, and the Beyonder.
They've really got to one-up part 2 with the convoluted and unnecessary subplots, and that's the only way I see it happening.
As long as every single new character mentioned above has a deeply personal relationship with Peter that goes way back even though nothing was ever mentioned in previous films then I'm in.