Didn't Spiderman 2 establish that Jameson's son was an astronaut? Maybe he (or somebody else) finds something in space? Heck, it could just fall to earth on its own a'la The Blob.
As for Harry, I'm thinking he'll be the the one in the Green Goblin suit. Not only did he find a bunch of his father's toys in the last movie, but they also showed he's not quite right in the head. Plus, it would be fitting if both he and Peter Parker had to battle their inner demons as well as each other. To me, this is where the last movie was already pointing. Putting some other, new character in the Green Goblin suit just seems so extraneous at this point.