Lloyd and Barbara's marriage is doomed to collapseAs seen in the episode Time Release Capsule, just after they started dating, they found amazing stuff in the time capsule. If they hadn't found the time capsule, then they would have gone their separate ways and forgotten about each other. Because they did find the time capsule, they couldn't bear to fight over the expensive objects inside. Their marriage is built on material goods. This is usually a sign of a not-so-healthy relationship. Ergo, there will eventually be a bitter feud and, ultimately, a split.
The Diffys stayed in the 21st Century when they turned around for CurtisAs seen in the episode Back To The Future (Not The Movie), the Diffys turned around for Curtis. Word of God says they would have stayed in the 21st Century.
Phil becomes his own ancestor...Which thus leads to the end of the pinky toe, causing a double Stable Time Loop.
Phil and Keely get marriedWhich probably messes up the Timey-Wimey Ball but who cares, Word of God says they ended up together. Those Time Police will go nuts.
Phil was a Time Lord.Yes, it's tiresomely predictable, but here is actual proof!
Seth becomes a mass school shooter.Theory 1: Since Seth leaves sometime in season 1 he could've moved to LA. Constance seems like the kind of person who would tell him he'd own people later in his life. He also could've wanted to start over at his new school so he died his hair blonde and changed his name to hide the humiliation of his past nerdyness. All of the stuff happened at home making him snap. Theory 2: Seth hangs out around Phil a lot in season 1. He accidentally found some of his time travel stuff and transported back to the 80s and become younger where Constance, wanting another child, took him in. She died his hair so he could look more like her actual son, named him Tate because he was too young to remember his name/care, and raised him as her own. When he's growing up he has 2 lives in his head which would be too confusing to almost anyone. He still had his previous intelligence (ex: Poetry and books about birds). From remembering 2 lives and everything at home or his tough 2nd upbringing combined with memories of everyone taunting him as a nerd in his 1st upbringing made him go mad/confused, thus the shooting and madness.