1. Well, the Calvin Universe is the result of George and Lorraine having made a different choice. Not only would poor Marty have to get used to having a different name, but there would also be one rather disappointed Uncle Dave.

2. The Parker Universe is the result of Marty making a different choice, by deciding to take Jennifer with him to the parking lot of Twin Pines Mall. Marty wouldn't've been Calvin Klein - as, with Jennifer being there, I think Lorraine would've shown much more restraint.

3. In the Newlyweds Universe, Marty pushes harder for Doc to show Clara the time machine - and just bring her back to 1985, with them. The ending wouldn't've been as suspenseful, but it would be a slightly happier ending for everyone.

4. If Doc was to just warn Marty about the race with Needles, and tell him about his terrible future - it might've been just as effective. It's basically what ends up happening in some of my AUs. However, Doc would've likely never met his Clara.

5. If Doc had just thrown out Marty's letter, that would've been very sad. Marty would've maybe tried to replace the plutonium pellet, as he tries to prevent the tragic death. He maybe could've made himself sick with radiation poisoning, as he wouldn't've know how to handle it carefully. He could also risk creating a time paradox, by peventing his younger self from going back to 1955.